Method of conditioning an internal combustion engine

ABSTRACT

A method of Improving the efficiency of a diesel engine provided with a source of diesel fuel includes the steps of: a) adding to the diesel fuel a reverse-micellar composition having an aqueous first disperse phase that includes a free radical initiator and a first continuous phase that includes a first hydrocarbon liquid, a first surfactant, and optionally a co-surfactant, thereby producing a modified diesel fuel; and b) operating the engine, thereby combusting the modified diesel fuel. The efficiency of a diesel engine provided with a source of diesel fuel and a source of lubricating oil can also be improved by modifying the lubricating oil by the addition of a stabilized nanoparticulate composition of cerium dioxide. The efficiency of a diesel engine can also be improved by adding to the diesel fuel a reverse-micellar composition that includes an aqueous disperse phase containing boric acid or a borate salt.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from: U.S. ProvisionalApplication Ser. No. 60/824,514, CERIUM-CONTAINING FUEL ADDITIVE, filedSep. 5, 2006; U.S. Provisional Application Ser. No. 60/911,159, REVERSEMICELLAR FUEL ADDITIVE COMPOSITION, filed Apr. 11, 2007; and U.S.Provisional Application Ser. No. 60/938,314, REVERSE MICELLAR FUELADDITIVE COMPOSITION, filed May 16, 2007, the disclosures of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to internal combustion engines and, inparticular, to the conditioning of such engines through the use of fueladditives to improve their efficiency.

BACKGROUND OF THE INVENTION

Diesel fuel ranks second only to gasoline as a fuel for internalcombustion engines. Trucks, buses, tractors, locomotives, ships, powergenerators, etc. are examples of devices that use diesel fuel. Passengercars and sport utility vehicles are another area of potential growth forthe use of diesel engines that can provide improved fuel efficiency,especially where high torque at relatively low rpm is desired.

Diesel fuel is principally a blend of petroleum-derived compounds calledmiddle distillates (heavier than gasoline but lighter than lube oil).Diesel fuel is designed to operate in a diesel engine, where it isinjected into the compressed, high-temperature air in the combustionchamber and ignites spontaneously. This differs from gasoline, which ispre-mixed with air and ignited in a gasoline engine by the spark plugs.D2 diesel fuel conforms to specification D 975 set by the AmericanSociety for Testing and Materials (ASTM).

Unlike gasoline engines that operate by spark ignition, diesel enginesemploy compression ignition. In order to avoid long ignition delaysresulting in rough engine operation, as well as to minimize misfiringand uneven or incomplete combustion which results in smoke in theexhaust gases that causes a major environmental problem, it is highlydesirable to improve the burning quality of diesel fuels to minimizeenvironmental pollutants such as hydrocarbons, carbon monoxide,particulate matter (commonly called soot), etc.

Cetane is an alkane molecule that ignites very easily under compression,so it is assigned a cetane number (CN) of 100. In general, the cetanenumber (CN) depends primarily on its hydrocarbon composition. Saturatedhydrocarbons, particularly those with straight, open chains, haverelatively high cetane numbers, whereas unsaturated hydrocarbons haverelatively low cetane numbers. All other hydrocarbons in diesel fuel areindexed to cetane as to how well they ignite under compression. Thecetane number therefore measures how quickly the fuel starts to burn(auto-ignites) under diesel engine conditions. Since there are hundredsof components in diesel fuel, with each having a different cetanequality, the overall cetane number of the diesel is the average cetanequality of all the components. Cetane improvers act to increase theeffective cetane number of the fuel.

It is necessary to recognize that the relationship between the CN ofdiesel fuel and its performance cannot be equated in any way to theoctane number of a gasoline and its performance in a spark-ignitionengine. Raising the octane number allows an increase in the compressionratio and thus provides increased power and fuel economy at a particularfuel load. In contrast, in diesel engines, the desired CN provides goodignition at high loads and low atmospheric temperature. High cetanefuels eliminate engine roughness and diesel knock, allow engines to bestarted at lower temperatures, provide faster engine warm-up withoutmisfiring or producing smoke and reduce formation of harmful deposits.On the other hand, too high cetane fuels can result in incompletecombustion and exhaust smoke due to too brief of an ignition delay whichdoes not allow proper mixing of the fuel and air.

Commercial diesel fuels have CN numbers of at least 40. The suitablediesel fuel has appropriate volatility, pour and cloud point, viscosity,gravity, flash point and contain only small but tolerable levels ofsulfur. It is also important that carbon, residue formation and ashcontent should be kept low.

During the normal course of operation, diesel engines often developcarbon deposits on the walls of their cylinders due to incompletecombustion of fuel. These deposits can increase engine wear and, becauseof friction induced by the deposits, decrease engine efficiency.Incomplete fuel combustion can also lead to the environmentally harmfulemission of particulate materials, also referred to as soot. Thus, fueladditives that increase fuel combustion, protect the cylinder walls ofdiesel engines, and decrease engine friction, resulting in greater fuelefficiency, are highly desirable.

Sanduja et al., U.S. Pat. No. 6,645,262, the disclosure of which isincorporated herein by reference, describes liquid hydrocarbon fuelconcentrates, including low-sulfur diesel fuel concentrates, thatinclude a suspension of particulate boric acid for the purpose ofincreasing lubricity and reducing engine wear.

Olah, U.S. Pat. No. 5,520,710, the disclosure of which is incorporatedherein by reference, describes diesel fuel additives that are dissolvedin the fuel and homogeneously distributed and include a dialkyl,alkyl-cycloalkyl, or dicycloalkyl ether compound together an alkyl ordialkyl peroxide compound for the purposes of enhancing cetane numbersand improving fuel combustion.

Peters et al., U.S. Pat. No. 6,158,397, the disclosure of which isincorporated herein by reference, describes a process for reducing sootin diesel engine exhaust gases wherein a fluid containing a peroxidecompound, preferably aqueous hydrogen peroxide, is separately fed intothe combustion chamber after the start of the injection and combustionof the fuel, preferably following the combustion phase.

Cunningham, U.S. Pat. No. 5,405,417, the disclosure of which isincorporated herein by reference, describes a fuel compositioncomprising a middle distillate base fuel having a sulfur content of lessthan 500 ppm and a clear cetane number in the range of 30 to 60, and aminor amount of at least one peroxy ester combustion improver such ast-butyl peroxyacetate dissolved therein.

Olsson et al., U.S. Pat. No. 5,105,772, the disclosure of which isincorporated herein by reference, describes a process for improvingcombustion in an engine that comprises: injecting a liquid compositionthat includes a peroxide or a peroxo compound into an engine combustionchamber, and passing a portion of the composition through the exhaustoutlet valve as the engine goes from the exhaust phase to the intakephase, the passing occurring during the step of injecting.

Mellovist et al., U.S. Pat. No. 4,359,969, the disclosure of which isincorporated herein by reference, describes a method of improving fuelcombustion that comprises: introducing a liquid composition consistingessentially of 1-10% hydrogen peroxide, 50-80% water, and 15-45% of aC₁-C₄ aliphatic alcohol, all by volume, in the form of fine dropletsinto the air intake manifold of an engine, where the droplets mix withair or fuel-air mixture prior to entering the combustion chamber.Preferably, the liquid composition also contains up to 5% of a thinlubricating oil and up to 1% of an anticorrosive.

Kracklaurer, U.S. Pat. No. 4,389,220, the disclosure of which isincorporated herein by reference, describes a method of conditioningdiesel engines in which a diesel engine is operated on a diesel fuelcontaining from about 20-30 ppm of dicyclopentadienyl iron for a periodof time sufficient to eliminate carbon deposits from the combustionsurfaces of the engine and to deposit a layer of iron oxide on thecombustion surfaces, which layer is effective to prevent further buildupof carbon deposits. Subsequently, the diesel engine is operated on amaintenance concentration of from about 10-15 ppm of dicyclopentadienyliron or an equivalent amount of a derivative thereof on a continuousbasis. The maintenance concentration is effective to maintain thecatalytic iron oxide layer on the combustion surfaces but insufficientto decrease timing delay in the engine. The added dicyclopentadienyliron may produce iron oxide on the engine cylinder surface (Fe₂O₃),which reacts with carbon deposits (soot) to form Fe and CO₂, therebyremoving the deposits. However, this method may accelerate the aging ofthe engine by formation of rust.

Valentine, et al., U.S. Patent Appl. Publ. No. 2003/0148235, thedisclosure of which is incorporated herein by reference, describespecific bimetallic or trimetallic fuel-borne catalysts for increasingthe fuel combustion efficiency. The catalysts reduce fouling of heattransfer surfaces by unburned carbon while limiting the amount ofsecondary additive ash which may itself cause overloading of particulatecollector devices or emissions of toxic ultra fine particles when usedin forms and quantities typically employed. By utilizing a fuelcontaining a fuel-soluble catalyst comprised of platinum and at leastone additional metal comprising cerium and/or iron, production ofpollutants of the type generated by incomplete combustion is reduced.Ultra low levels of nontoxic metal combustion catalysts can be employedfor improved heat recovery and lower emissions of regulated pollutants.However, fuel additives of this type, in addition to using the rare andexpensive metals such as platinum, can require several months before theengine is “conditioned”. By “conditioned” is meant that all the benefitsof the additive are not obtained until the engine has been operated withthe catalyst for a period of time. Initial conditioning may require 45days and optimal benefits may not be obtained until 60-90 days.Additionally, free metal may be discharged from the exhaust system intothe atmosphere, where it may subsequently react with living organisms.

Cerium dioxide is widely used as a catalyst in converters for theelimination of toxic exhaust emission gases and the reduction inparticulate emissions in diesel powered vehicles. Within the catalyticconverter, the cerium dioxide can act as a chemically active component,acting to release oxygen in the presence of reductive gases, as well asto remove oxygen by interaction with oxidizing species.

Cerium dioxide may store and release oxygen by the reversible processshown in equation 1.

2CeO₂⇄Ce₂O₃+½O₂   (eq. 1)

The redox potential between the Ce³⁺ and Ce⁴⁺ ions lies between 1.3 and1.8V and is highly dependent upon the anionic groups present and thechemical environment (CERIUM: A Guide to its Role in ChemicalTechnology, 1992 by Molycorp, Inc, Library of Congress Catalog CardNumber 92-93444)). This allows the foregoing reaction to easily occur inexhaust gases. Cerium dioxide may provide oxygen for the oxidation of COor hydrocarbons in an oxygen starved environment, or conversely mayabsorb oxygen for the reduction of nitrogen oxides (NOx) in an oxygenrich environment. Similar catalytic activity may also occur when ceriumdioxide is added as an additive to fuel, for example, diesel orgasoline. However, for this effect to be useful, the cerium dioxide mustbe of a particle size small enough, i.e., nanoparticulate (<100 nm), toremain in a stable dispersion in the fuel. In addition, as catalyticeffects depend on surface area, the small particle size renders thenanocrystalline material more effective as a catalyst. The incorporationof cerium dioxide in fuel serves not only to act as a catalyst to reducetoxic exhaust gases produced by fuel combustion, for example, by the“water gas shift reaction”

CO+H₂O→CO₂+H₂,

but also to facilitate the burning off of particulates that accumulatein the particulate traps typically used with diesel engines.

Cerium dioxide nanoparticles are particles that have a mean diameter ofless than 100 nm. For the purposes of this disclosure, unless otherwisestated, the diameter of a nanoparticle refers to its hydrodynamicdiameter, which is the diameter determined by dynamic light scatteringtechnique and includes molecular adsorbates and the accompanyingsolvation shell of the particle. Alternatively, the geometrical particlediameter bay be estimated using transmission electron micrography.

Vehicle on-board dosing systems that dispense cerium dioxide into thefuel before it enters the engine are known, but such systems arecomplicated and require extensive electronic control to feed theappropriate amount of additive to the fuel. To avoid such complexon-board systems, cerium dioxide nanoparticles can also be added to fuelat an earlier stage to achieve improved fuel efficiency. They can, forexample, be incorporated at the refinery, typically along withprocessing additives such as, for example, cetane improvers or added ata fuel distribution tank farm.

Cerium dioxide nanoparticles can also be added at a fuel distributioncenter, where it can be rack injected into large (˜100,000 gal) volumesof fuel or at a smaller fuel company depot, which would allowcustomization according to specified individual requirements. Inaddition, the cerium dioxide may be added at a filling station duringdelivery of fuel to a vehicle, which would have the potential advantageof improved stabilization of the particle dispersion.

Fuel additives, such as PuriNOx™ manufactured by Lubrizol Corporation,have been developed that are useful for the reduction of NOx andparticulate material emissions, however, the composition of these fueladditives often includes 15-20% water. This “emulsified” fuel additiveis commonly mixed with fuel at a level of 5-10%. The resulting highwater content can lead to a loss in engine power and lower fuel economy.Thus it would be desirable to formulate a fuel additive that affordedreduction in nitrogen oxide and particulate material emissions, whilesimultaneously maintaining optimum engine performance.

Cerium nanoparticles and the associated free radical initiators(incorporated into reverse micelles), as described below, can provide apossible solution to this problem. Cerium nanoparticles may form aceramic layer on the engine cylinders and moving parts essentiallyturning the engine into a catalytic device. Their catalytic efficiencyderives from the fact that they provide a source of oxygen atoms duringcombustion by undergoing reduction according to the equation (1). Thisresults in better fuel combustion and reduced levels of particulatematerial emissions. Additionally, when used as a fuel additive, thesenanoparticles can provide improved engine performance by reducing enginefriction. As an alternative mode of introduction, cerium dioxidenanoparticles can be added to the lube oil and act as a lubricityenhancing agent to reduce internal friction. This will also improve fuelefficiency.

Although substantially pure cerium dioxide nanoparticles arebeneficially included in fuel additives, it may be of further benefit touse cerium dioxide doped with components that result in the formation ofadditional oxygen vacancies being formed. For this to occur, the dopantshould be divalent or trivalent, i.e., a divalent or trivalent ion of anelement that is a rare earth metal, a transition metal or a metal ofGroup IIA, IIIB, VB, or VIB of the Periodic Table, and of a size thatallows incorporation of the ion in a lattice position within the surfaceor sub-surface region of the cerium dioxide nanoparticles. Thissubstitutional ion doping is preferred to interstitial ion doping, wherethe dopants occupy spaces between the normal lattice positions.

The following publications, the disclosures all of which areincorporated herein by reference, describe fuel additives containingcerium oxidic compounds.

Hawkins et al., U.S. Pat. No. 5,449,387, discloses a cerium (IV) oxidiccompound having the formula:

(H₂O)_(p)[CeO(A)₂(AH)_(n)]_(m)

in which the radicals A, which are the same or different, are each ananion of an organic oxyacid AH having a pK_(a) greater than 1, p is aninteger ranging from 0 to 5, n is a number ranging from 0 to 2, and m isan integer ranging from 1 to 12. The organic oxyacid is preferably acarboxylic acid, more preferably, a C₂-C₂₀ monocarboxylic acid or aC₄-C₁₂ dicarboxylic acid. The cerium-containing compounds can beemployed as catalysts for the combustion of hydrocarbon fuels.

Valentine et al., U.S. Pat. No. 7,063,729, discloses a low-emissionsdiesel fuel that includes a bimetallic, fuel-soluble platinum groupmetal and cerium catalyst, the cerium being provided as a fuel-solublehydroxyl oleate propionate complex.

Chopin et al., U.S. Pat. No. 6,210,451, discloses a petroleum-based fuelthat includes a stable organic sol that comprises particles of ceriumdioxide in the form of agglomerates of crystallites (preferred size 3-4nm), an amphiphilic acid system containing at least one acid whose totalnumber of carbons is at least 10, and an organic diluent medium. Thecontrolled particle size is no greater than 200 nm.

Birchem et al., U.S. Pat. No. 6,136,048, discloses an adjuvant fordiesel engine fuels that includes a sol comprising particles ofoxygenated compound having a d90 no greater than 20 nm, an amphiphilicacid system, and a diluent. The oxygenated metal compound particles areprepared from the reaction in solution of a rare earth salt such as acerium salt with a basic medium, followed by recovery of the formedprecipitate by atomization or freeze drying.

Lemaire et al., U.S. Pat. No. 6,093,223, discloses a process forproducing aggregates of ceric oxide crystallites by burning ahydrocarbon fuel in the presence of at least one cerium compound. Thesoot contains at least 0.1 wt. % of ceric oxide crystallite aggregates,the largest particle size being 50-10,000 angstroms, the crystallitesize being 50-250 angstroms, and the soot having an ignition temperatureof less than 400° C.

Hazarika et al., U.S. Patent Appl. Publ. No. 2003/0154646, discloses amethod of improving fuel efficiency and/or reducing fuel emissions of afuel burning apparatus, the method comprising dispersing at least oneparticulate lanthanide oxide, particularly cerium dioxide, in the fuel,wherein the particulate lanthanum oxide is coated with a surfactantselected from the group consisting of alkyl carboxylic anhydrides andesters having at least one C₁₀ to C₃₀ alkyl group.

Collier et al., U.S. Patent Appl. Publ. No. 2003/0182848, discloses adiesel fuel composition that improves the performance of diesel fuelparticulate traps and contains a combination of 1-25 ppm of metal in theform of a metal salt additive and 100-500 ppm of an oil-solublenitrogen-containing ashless detergent additive. The metal may be analkali metal, an alkaline earth metal, a metal of Group IVB, VIIB,VIIIB, IB, IIB, or any of the rare earth metals having atomic numbers57-71, especially cerium, or mixtures of any of the foregoing metals.

Collier et al., U.S. Patent Appl. Publ. No. 2003/0221362, discloses afuel additive composition for a diesel engine equipped with aparticulate trap, the composition comprising a hydrocarbon solvent andan oil-soluble metal carboxylate or metal complex derived from acarboxylic acid containing not more than 125 carbon atoms. The metal maybe an alkali metal, an alkaline earth metal, a metal of Group IVB, VIIB,VIIIB, IB, IIB, or a rare earth metal, including cerium, or mixtures ofany of the foregoing metals.

Caprotti et al., U.S. Patent Appl. Publ. No. 2004/0035045, discloses afuel additive composition for a diesel engine equipped with aparticulate trap. The composition comprises an oil-soluble oroil-dispersible metal salt of an acidic organic compound and astoichiometric excess of metal. When added to the fuel, the compositionprovides 1-25 ppm of metal, which is selected from the group consistingof Ca, Fe, Mg, Sr, Ti, Zr, Mn, Zn, and Ce.

Caprotti et al., U.S. Patent Appl. Publ. No. 2005/0060929, discloses adiesel fuel composition stabilized against phase separation thatcontains a colloidally dispersed or solubilized metal catalyst compoundand 5-1000 ppm of a stabilizer that is an organic compound having alipophilic hydrocarbyl chain attached to at least two polar groups, atleast one of which is a carboxylic acid or carboxylate group. The metalcatalyst compound comprises one or more organic or inorganic compoundsor complexes of Ce, Fe, Ca, Mg, Sr, Na, Mn, Pt, or mixtures thereof.

Wakefield, U.S. Pat. No. 7,169,196 B2, discloses a fuel comprisingcerium dioxide particles that have been doped with a divalent ortrivalent metal or metalloid that is a rare earth metal, a transitionmetal, or a metal of Group IIa, IIIB, VB, or VIB of the Periodic Table.

Caprotti et al., U.S. Patent Appl. Publ. No. 2006/0000140, discloses afuel additive composition that comprises at least one colloidal metalcompound or species and a stabilizer component that is the condensationproduct of an aldehyde or ketone and a compound comprising one or morearomatic moieties containing a hydroxyl substituent and a furthersubstituent chosen from among hydrocarbyl, —COOR, or —COR, R beinghydrogen or hydrocarbyl. The colloidal metal compound preferablycomprises at least one metal oxide, preferred oxides being iron oxide,cerium dioxide, or cerium-doped iron oxide.

Scattergood, International Publ. No. WO 2004/065529, discloses a methodfor improving the fuel efficiency of fuel for an internal combustionengine that comprises adding to the fuel cerium dioxide and/or dopedcerium dioxide and, optionally, one or more fuel additives.

Anderson et al., International Publ. No. WO 2005/012465, discloses amethod for improving the fuel efficiency of a fuel for an internalcombustion engine that comprises lubricating oil and gasoline, themethod comprising adding cerium dioxide and/or doped cerium dioxide tothe lubricating oil or the gasoline.

Cerium-containing nanoparticles can be prepared by a variety oftechniques known in the art. Regardless of whether the synthesizednanoparticles are made in a hydrophilic or hydrophobic medium, theparticles normally require a stabilizer to prevent undesirableagglomeration. The following publications, the disclosures all of whichare incorporated herein by reference, describe some of these synthetictechniques.

Talbot et al., U.S. Pat. No. 6,752,979, discloses a method of producingmetal oxide particles having nano-sized grains that consists of: mixinga solution containing one or more metal cations with a surfactant underconditions such that surfactant micelles are formed within the solution,thereby forming a micellar liquid; and heating the micellar liquid toremove the surfactant and form metal oxide particles having a disorderedpore structure. The metal cations are selected from the group consistingof cations from Groups 1A, 2A, 3A, 4A, 5A, and 6A of the Periodic Table,transition metals, lanthanides, actinides, and mixtures thereof.Preparations of particles of cerium dioxide and mixed oxides containingcerium and one or more other metals are included in the illustrativeexamples.

Chane-Ching et al., U.S. Pat. No. 6,271,269, discloses a process forpreparing storage-stable organic sols that comprises: reacting a basereactant with an aqueous solution of the salt of an acidic metal cationto form an aqueous colloidal dispersion containing excess hydroxyl ions;contacting the aqueous colloidal dispersion with an organic phasecomprising an organic liquid medium and an organic acid; and separatingthe resulting aqueous/organic phase mixture into an aqueous phase and aproduct organic phase. Preferred metal cations are cerium and ironcations. The colloidal particulates have hydrodynamic diameters, in therange of 50-2000 angstroms.

Chane-Ching, U.S. Pat. No. 6,649,156, discloses an organic solcontaining cerium dioxide particles that are made by a thermalhydrolysis process; an organic liquid phase; and at least oneamphiphilic compound chosen from polyoxyethylenated alkyl ethers ofcarboxylic acids, polyoxyethylenated alkyl ether phosphates, dialkylsulfosuccinates, and quaternary ammonium compounds. The water content ofthe sols may not be more than 1% . The mean crystallite size is about 5nm, while the particle agglomerates of these crystallites range in sizefrom 200 to 10 nm.

Chane-Ching, U.S. Pat. No. 7,008,965, discloses an aqueous colloidaldispersion of a compound of cerium and at least one other metal, thedispersion having a conductivity of at most 5 mS/cm and a pH between 5and 8.

Chane-Ching, U.S. Patent Appl. Publ. No. 2004/0029978 (abandoned Dec. 7,2005), discloses a surfactant formed from at least one nanoparticle thathas amphiphilic characteristics and is based on a metal oxide, hydroxideand/or oxyhydroxide, on the surface of which organic chains withhydrophobic characteristics are bonded. The metal is preferably selectedfrom among cerium, aluminum, titanium or silicon, and the alkyl chaincomprises 6-30 carbon atoms, or polyoxyethylene monoalkyl ethers ofwhich the alkyl chain comprises 8-30 carbon atoms and thepolyoxyethylene part comprises 1-10 ethyoxyl groups. The particle is anisotopic or spherical particle having an average diameter of 2-40 nm.

Blanchard et al., U.S. Patent Appl. Publ. No. 2006/0005465, discloses anorganic colloidal dispersion comprising: particles of at least onecompound based on at least one rare earth, at least one acid, and atleast one diluent, wherein at least 90% of the particles aremonocrystalline. Example 1 describes the preparation of a cerium dioxidecolloidal solution from cerium acetate and an organic phase,thatincludes Isopar hydrocarbon mixture and isostearic acid. The resultingcerium dioxide particles had a d₅₀ of 2.5 nm, and the size of 80% of theparticles was in the range of 1-4 nm.

Zhou et al., U.S. Pat. No. 7,025,943, discloses a method for producingcerium dioxide crystals that comprises: mixing a first solution of awater-soluble cerium salt with a second solution of alkali metal orammonium hydroxide; agitating the resulting reactant solution underturbulent flow conditions while concomitantly passing gaseous oxygenthrough the solution; and precipitating cerium dioxide particles havinga dominant particle size within the range of 3-100 nm. In Example 1, theparticle size is stated to be around 3-5 nm.

Noh et al., U.S. Patent Appl. Publ. No. 2004/0241070, discloses a methodfor preparing single crystalline cerium dioxide nanopowder comprising:preparing cerium hydroxide by precipitating a cerium salt in thepresence of a solvent mixture of organic solvent and water, preferablyin a ratio of about 0.1:1 to about 5:1 by weight; and hydrothermallyreacting the cerium hydroxide. The nanopowder has a particle size ofabout 30-300 nm.

Chan, U.S. Patent Appl. Publ. No. 2005/0031517, discloses a method forpreparing cerium dioxide nanoparticles that comprises: rapidly mixing anaqueous solution of cerium nitrate with aqueous hexamethylenetetramine,the temperature being maintained at a temperature no higher than about320° K. while nanoparticles form in the resulting mixture; andseparating the formed nanoparticles. The mixing apparatus preferablycomprises a mechanical stirrer and a centrifuge. In the illustrativeexample, the prepared cerium dioxide particles are reported to have adiameter of about 12 nm.

Ying et al., U.S. Pat. Nos. 6,413,489 and 6,869,584, disclose thesynthesis by a reverse micelle technique of nanoparticles that are freeof agglomeration and have a particle size of less than 100 nm and asurface area of at least 20 m²/g. The method comprises introducing aceramic precursor that includes barium alkoxide and aluminum alkoxide inthe presence of a reverse emulsion.

Illustrative example 9 of U.S. Pat. Nos. 6,413,489 and 6,869,584describes the inclusion of cerium nitrate in the emulsion mixture toprepare cerium-doped barium hexaaluminate particles, which werecollected by freeze drying and calcined under air to 500° C. and 800° C.The resulting particles had grain sizes of less than 5 nm and 7 nm at500° C. and 800° C., respectively. Illustrative example 10 describes thesynthesis of cerium-coated barium hexaaluminate particles. Followingcalcination, the cerium-coated particles had grain sizes of less than 4nm, 6.5 nm, and 16 nm at 500° C., 800° C., and 1100° C., respectively.

A related publication, Ying et al., U.S. Patent Appl. Publ. No.2005/0152832, discloses the synthesis, by a reverse micelle techniquewithin an emulsion having a 1-40% water content, of nanoparticles thatare free of agglomeration and have a particle size of less than 100 nm.The nanoparticles are preferably metal oxide particles, which can beused to oxidize hydrocarbons.

Illustrative Examples 9 and 10 of U.S. Patent Appl. Publ. No.2005/0152832 describe the preparation of, respectively, cerium-doped andcerium-coated barium hexaaluminate particles. Example 13 describes theoxidation of methane with the cerium-coated particles.

Hanawa et al., U.S. Pat. No. 5,938,837, discloses a method for preparingcerium dioxide particles, intended primarily for use as a polishingagent, that comprises mixing, with stirring, an aqueous solution ofcerous nitrate with a base, preferably aqueous ammonia, in such a mixingratio that the pH value of the mixture ranges from 5 to 10, preferably 7to 9, then rapidly heating the resulting mixture to a temperature of70-100° C., and maturing the mixture of cerous nitrate with a base atthat temperature to form the grains. The product grains are uniform insize and shape and have an average particle size of 10-80 nm, preferably20-60 nm.

European Patent Application EP 0208580, published 14 Jan. 1987, inventorChane-Ching, applicant Rhone Poulenc, discloses a cerium (IV) compoundcorresponding to the general formula

Ce(M)_(x)(OH)_(y)(NO₃)₂

wherein M represents an alkali metal or quaternary ammonium radical, xis between 0.01 and 0.2, y is such that y=4−z+x, and z is between 0.4and 0.7. A process for preparing a colloidal dispersion of the cerium(IV) compound produces particles with a hydrodynamic diameter betweenabout 1 nm and about 60 nm, suitably between about 1 nm and about 10 nm,and desirably between about 3 nm and 8 nm.

The doping of cerium dioxide with metal ions (reported as early as 1975)and the resultant dopant effects on the electronic and oxygen diffusionproperties are well described by Trovarelli, Catalysis by Ceria andRelated Materials, Catalytic Science Series, World Scientific PublishingCo., 37-46 (2002) and references cited therein.

S. Sathyamurthy et al., Nano Technology 16, (2005), pp 1960-1964,describes the reverse micellar synthesis of CeO₂ from cerium nitrate,using sodium hydroxide as the precipitating agent and n-octanecontaining the surfactant cetyltrimethylammonium bromide (CTAB) and thecosurfactant 1-butanol as the oil phase. The resulting polyhedralparticles had an average size of 3.7 nm, but the reaction would beexpected to proceed in low yield.

S. Seal et al., Journal of Nano Particle Research, (2002), p 438,describes the preparation from cerium nitrate and ammonium hydroxide ofnanocrystalline ceria particles for a high-temperatureoxidation-resistant coating using an aqueous microemulsion systemcontaining AOT as the surfactant and toluene as the oil phase. The ceriananoparticles formed in the upper oil phase of the reaction mixture hada particle size of 5 nm.

Pang et al., J. Mater. Chem., 12 (2002), pp 3699-3704, prepared Al₂O₃nanoparticles by a water-in-oil microemulsion method, using an oil phasecontaining cyclohexane and the non-ionic surfactant Triton X-114, and anaqueous phase containing 1.0 M AlCO₃. The resulting Al₂O₃ particles,which had a particle size of 5-15 nm, appeared to be distinctlydifferent from the hollow ball-shaped particles of submicron size madeby a direct precipitation process.

Cuif et al, U.S. Pat. No. 6,133,194, the disclosure of which isincorporated herein by reference, describes a process that comprisesreacting a metal salt solution containing cerium, zirconium, or amixture thereof, a base, optionally an oxidizing agent, and an additiveselected from the group consisting of anionic surfactants, nonionicsurfactants, polyethylene glycols, carboxylic acids, and carboxylatesalts, thereby forming a product. The product is subsequently calcinedat temperatures >500 C (which would effectively carbonize the claimedsurfactants).

Hazbun et al., U.S. Pat. No. 4,744,796, the disclosure of which isincorporated herein by reference, describes a microemulsion fuelcomposition that includes a hydrocarbon fuel and a cosurfactantcombination of t-butyl alcohol and at least one amphoteric, anionic,cationic, or nonionic surfactant. Preferred surfactants are fatty acidsor fatty acid mixtures.

Hicks et al., U.S. Patent Appl. Publ. No. 2002/0095859, the disclosureof which is incorporated herein by reference, describes additivecompositions for liquid hydrogen fuels that include one or moresurfactants selected from the group consisting of amphoteric, anionic,cationic, or nonionic surfactants, and optionally one or morecosurfactants selected from the group consisting of alcohols, glycols,and ethers.

As described previously, various methods and apparatus have beenreported for preparing cerium nanoparticles including those described byChane-Ching, et al., U.S. Pat. No. 5,017,352; Hanawa, et al., U.S. Pat.No. 5,938,837; Melard, et al., U.S. Pat. No 4,786,325; Chopin, et al.,U.S. Pat. No. 5,712,218; Chan, U.S. Patent Appl. Publ. No. 2005/0031517;and Zhou, et al., U.S. Pat. No. 7,025,943, the disclosures of which areincorporated herein by reference. However, current methods do not allowthe economical and facile preparation of cerium nanoparticles in a shortperiod of time at very high suspension densities (greater than 0.5molal, i.e., 9 wt. %) that are sufficiently small in size (less than 8nm in mean diameter), uniform in size frequency distribution(coefficient of variation [COV] of less than 15%, where COV is thestandard deviation divided by the mean diameter), and stable for manydesirable applications.

A typical chemical reactor that might be used to prepare cerium dioxideincludes a reaction chamber that includes a mixer (see, for example,FIG. 1 in Zhou et al. U.S. Pat. No. 7,025,943). A mixer typicallyincludes a shaft, and propeller or turbine blades attached to the shaft,and a motor that turns the shaft, such that the propeller is rotated athigh speed (1000 to 5000 rpm). The shaft can drive a flat blade turbinefor good meso mixing (micro scale) and a pitched blade turbine for macromixing (pumping fluid through out the reactor).

Such a device is described in Antoniades, U.S. Pat. No. 6,422,736,entitled “Scaleable Device Impeller Apparatus For Preparing SilverHalide Grains.” This type of reactor is useful for fast reactions suchas that shown by the equation below, wherein the product, AgCl, is acrystalline material having a diameter on the order of several hundrednanometers up to several thousand nanometers.

AgNO₃+NaCl→AgCl+NaNO₃

Cerium dioxide particles prepared using this type of mixing are oftentoo large to be useful for certain applications. It is highly desirableto have the smallest cerium dioxide particles possible as theircatalytic propensity (ability to donate oxygen to a combustion system,i.e., equation 1) increases with decreasing particle size, especiallyfor particles having a mean diameter of less than 10 nm.

A schematic example of a batch reactor that can be used to producecerium dioxide nanoparticles is shown in FIG. 1. The reactor (10)includes inlet ports (11, and 12) for adding reactants, a propeller,shaft, and motor, 15, 14, and 13, for mixing. The reaction mixture 18 iscontained in a reactor vessel 16. Addition of reactants, such as ceriumnitrate, an oxidant, and hydroxide ion, can result in the formation ofnanoparticles. The particles initially form as very small nuclei. Mixingcauses the nuclei to circulate, shown by the dashed arrows (17) inFIG. 1. As the nuclei continuously circulate through the reactive mixingregime they grow (increase in diameter) as they incorporate freshreactants. Thus, after an initial steady state concentration of nucleiis formed, this nuclei population is subsequently grown into largerparticles in a continuous manner. This nucleation and growth process isnot desirable if one wishes to limit the final size of the particleswhile still maintaining a high particle suspension density. Such a batchreactor is not useful for producing a high yield (greater than 1 molal)of cerium dioxide nanoparticles that are very small, for example, lessthan 10 nm in a reasonably short reaction time (for example, less than60 minutes).

An example of this nucleation and growth process applied to the aqueousprecipitation of CeO₂ is the work of Zhang et al., J. Appl. Phys., 95,4319 (2004) and Zhang, et al., Applied Physics Letters, 80, 127 (2002).Using cerium nitrate hexahydrate as the cerium source (very dilute at0.0375M) and 0.5 M hexamethylenetetramine as the ammonia precursor, 2.5to 4.25 nm cerium dioxide particles are formed in times that are lessthan 50 minutes. These particles are subsequently grown to 7.5 nm orgreater using reaction times on the order of 250 minutes (or 600 minutesdepending upon growth conditions). The limitations of particle size,concentration and reaction time would exclude this process fromconsideration as an economically viable route to bulk commercialquantities of CeO₂ nanoparticles.

I. H. Leubner, Current Opinion in Colloid and Interface Science, 5,151-159 (2000), Journal of Dispersion Science and Technology, 22,125-138 (2001) and ibid. 23, 577-590 (2002), and references citedtherein, provides a theoretical treatment that relates the number ofstable crystals formed with molar addition rate of reactants, solubilityof the crystals and temperature. The model also accounts for the effectsof diffusion, kinetically controlled growth processes, Ostwald ripeningagents and growth restrainers/stabilizers on crystal number. High molaraddition rates, low temperatures, low solubility, and the presence ofgrowth restrainers all favor large numbers of nuclei and consequentlysmaller final grain or particle size

In contrast to batch reactors, colloid mills typically have flat bladeturbines turning at 10,000 rpm, whereby the materials are forced througha screen whose holes can vary in size from fractions of a millimeter toseveral millimeters. Generally, no chemical reaction is occurring, butonly a change in particle size. In certain cases, particle size andstability can be controlled thermodynamically by the presence of asurfactant. For example, Langer et al., in U.S. Pat. No. 6,368,366 andU.S. Pat. No. 6,363,237, incorporated herein by reference, describe anaqueous microemulsion in a hydrocarbon fuel composition made under highshear conditions. However, the aqueous particle phase (the discontinuousphase in the fuel composition) has a large size, on the order of 1000nm.

Colloid mills are not useful for reducing the particle size of largecerium dioxide particles because the particles are too hard to besheared by the mill in a reasonable amount of time. The preferred methodfor reducing large, agglomerated cerium dioxide particles from themicron size down into the nanometer size is milling for several days ona ball mill in the presence of a stabilizing agent. This is a timeconsuming, expensive process that invariably produces a widedistribution of particle sizes. Thus, there remains a need for aneconomical and facile method to synthesize large quantities (at highsuspension densities) of very small nanometric particles of ceriumdioxide with a uniform size distribution.

Aqueous precipitation may offer a convenient route to ceriumnanoparticles. However, to be useful as a fuel-borne catalyst for fuels,cerium dioxide nanoparticles must exhibit stability in a nonpolar medium(for example, diesel fuel). Most stabilizers used to preventagglomeration in an aqueous environment are ill suited to the task ofstabilization in a nonpolar environment. When placed in a nonpolarsolvent, such particles tend to immediately agglomerate and,consequently, lose some, if not all, of their desirable nanoparticulateproperties. Thus it would be desirable to form stable cerium dioxideparticles in an aqueous environment, retain the same stabilizer on theparticle surface, and then be able to transfer these particles to anonpolar solvent, wherein the particles would remain stable and form ahomogeneous mixture. In this simplified and economical manner, one couldeliminate the necessity for changing surface stabilizer's affinity frompolar to non-polar. Changing stabilizers can involve a difficultdisplacement reaction or separate, tedious isolation-redispersal methods(for example, precipitation and subsequent redispersal with the newstabilizer using ball milling).

Thus, there remains a need for an efficient and economical method tosynthesize stable cerium dioxide nanoparticles in a polar, aqueousenvironment, and then transfer these particles to a non-polarenvironment wherein a stable homogeneous mixture is formed.

For some applications, it may even be desirable to have some relativelylow level of water present during the combustion process of an internalcombustion engine. The previously mentioned, Hicks et al., U.S. PatentAppl. Publ. No. 2002/0095859 suggests that as little as 5 to 95 ppmwater (as a microemulsion) improves hydrocarbon fuel combustion via thereduction of cyclic dispersion (variability between compression cycles).

Water added to diesel fuel is thought to improve combustion in threeways:

-   -   1. Water promotes a finer, more even spray pattern for more        complete combustion.    -   2. Water lowers the combustion temperature to reduce nitrous        oxide emissions (flame temperature of 2900° F.).    -   3. Water delays combustion slightly to reduce particulate        emissions.

J. Ying et al in WO 98/18884 describe a thermally and d temporallystable water-in-fuel emulsion having micelle size of <100 nm andincluding water in an amount of at least 8 wt. percent. As there was noattendant measurement of engine power, the claimed 85-90% reductions inparticulate emissions may have been an artifact of the loss of enginepower and thus been an unacceptable trade-off of power for emissionsreduction. Fuel additives that include cerium dioxide nanoparticles,wherein nanoparticles typically have a mean diameter of 100 nm or less,stabilized with a surfactant, such as sodium dodecyl succinate, andoptionally containing copper, are known. These types of fuel additivesalso have a long conditioning period.

The use of cerium nanoparticles to provide a high temperature oxidationresistant coating has been reported, for example, see “Synthesis Of NanoCrystalline Ceria Particles For High Temperature Oxidization ResistantCoating,” S. Seal et al., Journal of Nanoparticle Research, 4, 433-438(2002). The deposition of cerium dioxide on various surfaces has beeninvestigated, including Ni, chromia and alumina alloys, and stainlesssteel and on Ni, and Ni—Cr coated alloy surfaces. It was found that acerium dioxide particle size of 10 nm or smaller is desirable. Ceriaparticle incorporatiion subsequently inhibits oxidation of the metalsurface.

In addition, the extent to which CeO₂ can act as a catalytic oxygenstorage material, described by equation 1, is governed in part by theCeO₂ particle size. At 20 nm particle sizes and below, the latticeparameter increases dramatically with decreasing crystallite size (up to0.45% at 6 nm, see for example Zhang, et al., Applied Physics Letters,80 1, 127 (2002)). The associated size-induced lattice strain isaccompanied by an increase in surface oxygen vacancies that results inenhanced catalytic activity. This (inverse) size dependent activityprovides not only for more efficient fuel cells, but better oxidativeproperties when used in the combustion of petroleum fuels.

Henly, U.S. Patent Appl. Publ. No. 2005/0005506, the disclosure of whichis incorporated herein by reference, has described a distillate fueladditive composition, including calcium sulfonate detergent, asuccinimide dispersant, and an organomanganese compound. The organicmanganese compound, along with other compounds, acts to improve thecleanliness of the fuel system.

Rim, U.S. Pat. No. 6,892,531, the disclosure of which is incorporatedherein by reference, describes an engine lubricating oil composition fora diesel engine that includes a lubricating oil and 0.05-10 wt. % of acatalyst additive combprisng cerium carboxylate.

As described above, currently available fuel additives have improved theperformance of diesel engines; however further improvements are stillneeded. It would be desirable to formulate a fuel additive for dieselengines that provides: improved fuel combustion while maintaining enginepower while simultaneously reducing, reduced PM emissions. In addition,protection of engines from wear, reduced engine friction, greaterlubricity, with improved fuel efficiency would be tremendouslybeneficial. It would also be desirable to provide one or more of thesefeatures without requiring a long conditioning period.

SUMMARY OF THE INVENTION

The present invention is directed to a method of improving theefficiency of a diesel engine provided with a source of diesel fuel,wherein the method comprises the steps of: a) adding to the diesel fuela reverse micellar composition comprising an aqueous first dispersephase that includes a free radical initiator and a first continuousphase that includes a first hydrocarbon liquid, a first surfactant, andoptionally a co-surfactant, thereby producing a modified diesel fuel;and b) operating the engine, thereby combusting the modified dieselfuel.

The present invention is further directed to a method of improving theefficiency of a diesel engine provided with a source of diesel fuel anda source of lubricating oil, wherein the method comprises the steps of:a) adding to the diesel fuel a reverse micellar composition comprisingan aqueous first disperse phase that includes a free radical initiatorand a first continuous phase that includes a first hydrocarbon liquidand a first surfactant, thereby producing a modified diesel fuel; b)adding to the lubricating oil a stabilized nanoparticulate compositionof cerium dioxide, thereby producing a modified lubricating oil; and c)operating the engine, thereby combusting the modified diesel fuel andlubricating the engine with the modified lubricating oil.

The present invention is also directed to a method of improving theefficiency of a diesel engine provided with a source of diesel fuel,wherein the method comprises the steps of: a) adding to the diesel fuela first reverse micellar composition that includes an aqueous firstdisperse phase comprising boric acid or a borate salt and a firstcontinuous phase that includes a first hydrocarbon liquid, a firstsurfactant, and optionally a co-surfactant; and b) operating the engine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a conventional batch reactorfor forming cerium dioxide nanoparticles.

FIG. 2A shows a schematic exploded view of a colloid mill reactor thatmay be used in the invention.

FIG. 2B shows a partial view of a colloid mill reactor that may be usedin the invention.

FIG. 2C shows a schematic exploded view of another type of colloid millreactor that may be used in the invention.

FIG. 3 shows a schematic representation of a continuous reactor forforming very small cerium nanoparticles.

FIG. 4 shows the size distribution of the cerium dioxide particlesprepared in Example 1.

FIG. 5 shows a transmission electron micrograph of a dried-down sampleof the cerium dioxide particles of Example 1.

FIG. 6 shows an X-ray powder diffraction spectrum of cerium dioxidenanoparticles prepared in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The preparation of cerium dioxide nanoparticles is described inco-pending, commonly assigned application Ser. No. ______, METHOD OFPREPARING CERIUM DIOXIDE NANOPARTICLES, filed Sep. ______, 2007, thedisclosure of which is incorporated herein by reference.

Cerous ion reacts, in the presence of hydroxide ion, to form ceriumhydroxide. The reaction vessel is then heated to convert ceriumhydroxide to cerium dioxide. The temperature in the reaction vessel ismaintained between about 50° C. and about 100° C., more preferably about65-75° C., most preferably about 70° C. Time and temperature can betraded off, higher temperatures typically reducing the time required forconversion of the hydroxide to the oxide. After a period at theseelevated temperatures, on the order of about 1 hour or less and suitablyabout 0.5 hour, the cerium hydroxide is converted to cerium dioxide andthe temperature of the reaction vessel is lowered to about 15-25° C.Subsequently, the cerium dioxide nanoparticles are concentrated, and theunreacted cerium and waste by-products such as ammonium nitrate areremoved, most conveniently for example, by diafiltration.

In one aspect of the present invention, a method of making ceriumdioxide nanoparticles includes providing an aqueous reaction mixturecomprising cerous ion, hydroxide ion, a stabilizer, and an oxidant at atemperature effective to generate small nuclei size, and achievesubsequent oxidation of cerous ion to ceric ion so that these particlescan be grown into nanometric cerium dioxide. The reaction mixture issubjected to mechanical shearing, preferably by causing it to passthrough a perforated screen, thereby forming a suspension of ceriumdioxide nanoparticles having a mean hydrodynamic diameter in the rangeof about 2 nm to about 15 nm. While the particle diameter can becontrolled within the range of 2 nm to 15 nm, preferably the ceriumdioxide nanoparticles have a mean hydrodynamic diameter of about 10 nmor less, more preferably about 8 nm or less, most preferably, about 6nm. Desirably, the nanoparticles comprise one or at most two primarycrystallites per particle edge, each crystallite being on average 2.5 nm(approximately 5 unit cells). Thus, the resulting nanoparticle sizefrequency in substantially monodisperse, i.e., having a coefficient ofvariation (COV) less than 15%, where the COV is defined as the standarddeviation divided by the mean.

Mechanical shearing includes the motion of fluids upon surfaces such asthose of a rotor, which results in the generation of shear stress.Particularly, the laminar flux on a surface has a zero velocity, andshear stress occurs between the zero-velocity surface and thehigher-velocity flow away from the surface.

In one embodiment, the current invention employs a colloid mill, whichis normally used for milling micro emulsions or colloids, as a chemicalreactor to produce cerium dioxide nanoparticles. Examples of usefulcolloid mills include those described by Korstvedt, U.S. Pat. No.6,745,961 and U.S. Pat. No. 6,305,626, the disclosures of which areincorporated herein by reference.

A colloid mill, referred to as a Silverson mill, is depicted in U.S.Pat. No. 5,552,133, the disclosure of which is incorporated herein byreference. FIG. 2A schematically represents a colloid mill reactor,according to the present invention, that includes reactant inlet jets 34and 35. The depicted colloid mill reactor has a rotating shaft 30 thatis connected to a paddle blade rotor 31. The rotor is received in acup-shaped screen stator 32, which has perforations 36 and encloses thereaction chamber 37. The stator is mounted on a housing, 33, fitted withinlet jets 34 and 35. The inlet jets 34 and 35 extend into the housing33 to the bottom of the perforated screen stator 32 into the reactionchamber 37. A plate (not shown) forms a top to the screen stator 32. Thereactants are introduced via jets 34 and 35 into the reaction chamber.The colloidal mill reactor is enclosed in a reaction vessel 38, whichmay be submerged in a constant temperature bath (not shown).

During the stirring of the reaction mixture by rotation of the rotorshaft, the shaft rotation causes mechanical shearing of the reactionmixture between the flat faces (35) of the paddle rotor and the innercylindrical surface of the stator. Cerium hydroxide particles initiallyformed in the reaction chamber are forced through the perforations inthe screen and into the surrounding reaction vessel.

Various factors influence the mean diameter size and yield of theproduct cerium dioxide particles. Factors include reactant ratios, therotor speed, the “gap” of the mill, which can be defined as the spacebetween the rotor 31 and stator 32, and the size of the perforations 36of the stator.

Typical rotor speeds are 5000 to 7500 rpm; however, at very high reagentconcentrations (about 1 Molal or greater) rotor speeds of greater than7500 rpm, such as 10,000 rpm, are preferred. It is desirable to keep thegap spacing as small as possible, typically about 1 mm to about 3 mm,consistent with a low back pressure in the colloid chamber, which allowsa facile passage of the particles through the perforations of thestator. In one embodiment, the perforations of the screen have a meandiameter of preferably about 0.5 mm to about 5 mm.

FIG. 2B shows a partial view of the reactor, including the inlet jets 34and 35 and the base of the reaction chamber 33A. In one embodiment, theinlet jets 34 and 35 are substantially flush with the bottom of thereaction chamber 33A.

FIG. 2C shows a schematic representation of a modification of the devicedescribed above, wherein the inlet jets, 34 and 35, extend into thereaction chamber from the top of the mill, instead of the bottom of themill. Reactants are introduced into the reaction chamber by means of thereaction inlet(s) and the reaction mixture is stirred. Desirably, thereactants include an aqueous solution of cerous ion, for example cerousnitrate; an oxidant such as hydrogen peroxide or molecular oxygen; and astabilizer, such as 2-[2-(2-methoxyethoxy)ethoxy]acetic acid.Typically,a two-electron oxidant, such as peroxide, is present,preferably in at least one-half the molar concentration of the ceriumion. The hydroxide ion concentration is preferably at least twice, morepreferably three times, the molar cerium concentration.

Initially, the reaction chamber is maintained at a temperaturesufficiently low to generate small cerous hydroxide nuclei size, whichcan be grown into nanometric cerium dioxide particles after a subsequentshift to higher temperatures, resulting in conversion of the cerous ioninto the ceric ion state. Initially, the temperature is suitably about25° C. or less, preferably about 20° C., more preferably about 15° C. Inone embodiment, the temperature is about 10-20° C.

In one embodiment, a source of cerous ion, a nanoparticle stabilizer,and an oxidant is placed in the reactor and a source of hydroxide ion,such as ammonium hydroxide, is rapidly added with stirring, preferablyover a time period of about 90 seconds or less, more preferably about 20seconds or less, even more preferably about 15 seconds or less. In analternative embodiment, a source of hydroxide ion and an oxidant isplaced in the reactor, and a source of cerous ion is added over a periodof about 15 seconds. In a third and preferred embodiment, thestabilizers are placed in the reaction vessel, and the cerous nitrate issimultaneously introduced into the reaction chamber with a separate jetof ammonium hydroxide at the optimum molar stoichiometric ratio of 2:1or 3:1 OH:Ce.

Cerous ion reacts in the presence of hydroxide ion to form ceriumhydroxide, which can be converted by heating to cerium dioxide. Thetemperature in the reaction vessel is maintained between about 50° C.and about 100° C., preferably about 65-90° C., more preferably about 80°C. After a period of time at these elevated temperatures, preferablyabout 1 hour or less, more preferably about 0.5 hour, the ceriumhydroxide has been substantially converted to cerium dioxide, and thetemperature of the reaction vessel is lowered to about 15-25° C. Thetime and temperature variables may be traded off, higher temperaturesgenerally requiring shorter reaction times. The suspension of ceriumdioxide nanoparticles is concentrated, and the unreacted cerium andwaste by-products such as ammonium nitrate are removed, which may beconveniently accomplished by diafiltration.

The nanoparticle stabilizer is a critical component of the reactionmixture. Desirably, the nanoparticle stabilizer is water soluble andforms weak bonds with cerium ion. K_(BC) represents the binding constantof the nanoparticle stabilizer to cerium ion in water. Log K_(BC) forthe nitrate ion is 1 and for hydroxide ion is 14. Most desirably, log(K_(BC)) lies within this range, preferably towards the bottom of thisrange. Useful nanoparticle stabilizers include alkoxysubstitutedcarboxylic acids, α-hydroxyl carboxylic acids, pyruvic acid and smallorganic polyacids such as tartaric acid and citric acid. Examples ofethoxylated carboxylic acids include 2-(methoxy)ethoxy acetic acid and2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEA). Among the α-hydroxycarboxylic acids, examples include lactic acid, gluconic acid and2-hydroxybutanoic acid. Polyacids include ethylenediaminetetraaceticacid (EDTA), tartaric acid, and citric acid. Combinations of compoundswith large K_(BC) such as EDTA with weak K_(BC) stabilizers such aslactic acid are also useful at particular ratios. Large K_(BC)stabilizers such as gluconic acid may be used at a low level or withweak K_(BC) stabilizers such as lactic acid.

In one desirable embodiment, the nanoparticle stabilizer includes acompound of formula (Ia). In formula (Ia), R represents hydrogen, or asubstituted or unsubstituted alkyl group or aromatic group such as, forexample, a methyl group, an ethyl group or a phenyl group. Morepreferably, R represents a lower alkyl group such as a methyl group. R¹represents hydrogen or a substituent group such as an alkyl group. Informula (Ia), n represents an integer of 0-5, preferably 2. In formula(Ia), Y represents H or a counterion, such as an alkali metal, forexample Na⁺ or K⁺. The stabilizer binds to the nanoparticles andprevents agglomeration of the particles and the subsequent formation oflarge clumps of particles.

R—O—(CH₂CH₂O)_(n)CHR¹CO₂Y   (Ia)

In another embodiment, the nanoparticle stabilizer is represented byformula (Ib), wherein each R² independently represents a substituted orunsubstituted alkyl group or a substituted or unsubstituted aromaticgroup. X and Z independently represent H or a counterion such as Na⁺ orK⁺ and p is 1 or 2.

XO₂C(CR²)_(p)CO₂Z   (Ib)

Useful nanoparticle stabilizers are also found amongα-hydroxysubstituted carboxylic acids such as lactic acid or even thepolyhydroxysubstituted acids such as gluconic acid.

Preferably, the nanoparticle stabilizer does not include the elementsulfur, since sulfur-containing materials may be undesirable for certainapplications. For example, if the cerium dioxide particles are includedin a fuel additive composition, the use of a sulfur-containingstabilizer such as AOT may result in the undesirable emission of oxidesof sulfur after combustion.

The size of the resulting cerium dioxide particles can be determined bydynamic light scattering, a measurement technique for the determinationof a particle's hydrodynamic diameter. The hydrodynamic diameter (cf. B.J. Berne and R. Pecora, “Dynamic Light Scattering: With Applications toChemistry, Biology and Physics”, John Wiley and Sons, NY 1976 and“Interactions of Photons and Neutrons with Matter”, S. H. Chen and M.Kotlarchyk, World Scientific Publishing, Singapore, 1997), which isslightly larger than the geometric diameter of the particle, includesboth the native particle size and the solvation shell surrounding theparticle. When a beam of light passes through a colloidal dispersion,the particles or droplets scatter some of the light in all directions.When the particles are very small compared with the wavelength of thelight, the intensity of the scattered light is uniform in all directions(Rayleigh scattering). If the light is coherent and monochromatic as,for example, from a laser, it is possible to observe time-dependentfluctuations in the scattered intensity, using a suitable detector suchas a photomultiplier capable of operating in photon counting mode. Thesefluctuations arise from the fact that the particles are small enough toundergo random thermal (Brownian) motion, and the distance between themis therefore constantly varying. Constructive and destructiveinterference of light scattered by neighboring particles within theilluminated zone gives rise to the intensity fluctuation at the detectorplane which, because it arises from particle motion, containsinformation about this motion. Analysis of the time dependence of theintensity fluctuation can therefore yield the diffusion coefficient ofthe particles from which, via the Stokes Einstein equation and the knownviscosity of the medium, the hydrodynamic radius or diameter of theparticles can be calculated.

In another aspect of the invention, a continuous process for producingsmall cerium dioxide nanoparticles, that is, particles having a meandiameter of less than about 10 nm, includes combining cerous ion, anoxidant, a nanoparticle stabilizer, and hydroxide ion within acontinuous reactor, into which reactants and other ingredients arecontinuously introduced, and from which product is continuously removed.Continuous processes are described, for example, in Ozawa, et al., U.S.Pat. No. 6,897,270; Nickel, et al., U.S. Pat. No. 6,723,138; Campbell,et al., U.S. Pat. No. 6,627,720; Beck, U.S. Pat. No. 5,097,090; andByrd, et al., U.S. Pat. No. 4,661,321; the disclosures of which areincorporated herein by reference.

A solvent such as water is often employed in the process. The solventdissolves the reactants, and the flow of the solvent can be adjusted tocontrol the process. Advantageously, mixers can be used to agitate andmix the reactants.

Any reactor that is capable of receiving a continuous flow of reactantsand delivering a continuous flow of product can be employed. Thesereactors may include continuous-stirred-tank reactors, plug-flowreactors, and the like. The reactants required to carry out thenanoparticle synthesis are preferably charged to the reactor in streams;i.e., they are preferably introduced as liquids or solutions. Thereactants can be charged in separate streams, or certain reactants canbe combined before charging the reactor.

Reactants are introduced into the reaction chamber provided with astirrer through one or more inlets. Typically, the reactants include anaqueous solution of cerous ion, for example, cerous nitrate; an oxidantsuch as hydrogen peroxide or molecular oxygen, including ambient air;and a stabilizer, such as 2-[2-(2-methoxyethoxy)ethoxy]acetic acid. Atwo-electron oxidant such as hydrogen peroxide is present, preferably inat least one-half the molar concentration of the cerium ion.Alternatively, molecular oxygen can be bubbled through the mixture. Thehydroxide ion concentration is preferably at least twice the molarcerium concentration.

In one embodiment of the present invention, a method of forming smallcerium dioxide nanoparticles includes the step of forming a firstaqueous reactant stream that includes cerous ion, for example, ascerium(III)nitrate, and an oxidant. Suitable oxidants capable ofoxidizing Ce(III) to Ce(IV) include, for example, hydrogen peroxide ormolecular oxygen. Optionally, the first reactant stream also includes ananoparticle stabilizer that binds to cerium dioxide nanoparticles,thereby preventing agglomeration of the particles. Examples of usefulnanoparticle stabilizers were mentioned above.

The method further includes a step of forming a second aqueous reactantstream that includes a hydroxide ion source, for example, ammoniumhydroxide or potassium hydroxide. Optionally, the second reactant streamfurther includes a stabilizer, examples of which were describedpreviously. At least one of the first or second reactant streams,however, must contain a stabilizer.

The first and second reactant streams are combined to form a reactionstream. Initially, the temperature of the reaction stream is maintainedsufficiently low to form small cerous hydroxide nuclei. Subsequently thetemperature is raised so that oxidation of Ce(III) to Ce(IV) occurs inthe presence of the oxidant, and the hydroxide is converted to theoxide, thereby producing a product stream that includes cerium dioxide.The temperature for conversion from the hydroxide to the oxide ispreferably in the range of about 50-100° C., more preferably about60-90° C. In one embodiment, the first and second reactant streams arecombined at a temperature of about 10-20° C., and the temperature issubsequently increased to about 60-90° C.

Desirably, cerium dioxide nanoparticles in the product stream areconcentrated, for example, by diafiltration techniques using one or moresemi-porous membranes. In one embodiment, the product stream includes anaqueous suspension of cerium dioxide nanoparticles that is reduced to aconductivity of about 3 mS/cm or less by one or more semi-porousmembranes.

A schematic representation of a continuous reactor suitable for thepractice of the invention is depicted in FIG. 3. The reactor 40 includesa first reactant stream 41 containing aqueous cerium nitrate. An oxidantsuch as hydrogen peroxide is added to the reactant stream by means ofinlet 42, and the reactants are mixed by mixer 43 a. To the resultingmixture is added stabilizer via inlet 45, followed by mixing by mixer 43b. The mixture from mixer 43 b then enters mixer 43 c, where it iscombined with a second reactant stream containing ammonium hydroxidefrom inlet 44. The first and second reactant streams are mixed using amixer 43 c to form a reaction stream that may be subjected to mechanicalshearing by passing it through a perforated screen. In a furtherembodiment, mixer 43 c comprises a colloid mill reactor, as describedpreviously, that is provided with inlet ports for receiving the reactantstreams and an outlet port 45. In a further embodiment, the temperatureof the mixer 43 c is maintained at a temperature in the range of about10° C. to about 25° C.

The mixture from 43 c enters a reactor tube 45 that is contained in aconstant temperature bath 46 that maintains tube 45 at a temperature ofabout 60-90° C. Cerium nanoparticles are formed in the reactor tube 45,which may include a coil 50. The product stream then enters one or morediafiltration units 47, wherein the cerium nanoparticles areconcentrated using one or more semi-porous membranes. One or morediafiltration units may be connected in series to achieve a single passconcentration of product, or the units may placed in parallel for veryhigh volumetric throughput. The diafiltration units may be disposed bothin series and parallel to achieve both high volume and rapid throughput.Concentrated cerium nanoparticles exit the diafiltration unit via exitport 49, and excess reactants and water are removed from thediafiltration unit 47 via exit port 48. In an alternative embodiment,stabilizer may be added to the second reactant stream via port 51 ratherthan to the first reactant stream via port 45.

In one embodiment of the invention, the product stream of concentratedcerium nanoparticles exiting the diafiltration unit 47 is combined witha stream that includes a nonpolar solvent and at least one surfactant,wherein the surfactant is chosen so that a reverse micelle is formed inthe emulsion, as described below.

The use of a continuous process for producing cerium dioxidenanoparticles allows better control of the production of particle nucleiand their growth relative to that afforded by batch reactors. The nucleisize can be controlled by the initial reagent concentration,temperature, and the ratio of nanoparticle stabilizer to reagentconcentrations. Small nuclei are favored by low temperatures, less thanabout 20° C., and high ratios of nanoparticle stabilizer to reagentconcentrations. In this way, very small cerium dioxide nanoparticleshaving a mean hydrodynamic diameter of less than about 10 nm can beproduced in an economical manner.

It may be possible to use some of the aqueous precipitation medium inwhich cerium dioxide particles are typically formed to subsequentlyenhance the activity of the nanoparticles. When a mixture, includingcerium nanoparticles and a small amount of water, undergoes combustionin the presence of air and fuel in a diesel engine, flame temperaturesmay reach levels as high as 900° C. (1652° F.). At these hightemperatures, reduction of cerium and production of oxygen according toequation 1 is very efficient. Additionally, at these elevatedtemperatures superheated steam can be generated from the water. This notonly will increase the compression ratio, resulting in higher engineefficiency, but will also result in the separation of the fuel wavefront into many, very small, high surface area droplets. This allowsbetter mixing of the air-fuel regions, which enables the cerium dioxideparticles to provide oxygen to the fuel more readily, resulting in morecomplete fuel combustion. This in turn increases engine performancewhile simultaneously reducing particulate matter emissions. Ifsufficient water is present, the combustion temperature will be loweredsomewhat, and may also reduce levels of nitrogen oxide (NO_(x))production, which is greatest at higher temperatures. However atsufficiently high levels of water, the combustion temperature can belowered to the point at which engine power is reduced. This phenomenoncan be offset by replacing some of the water in the aqueous phase with awater-soluble cetane improver such as hydrogen peroxide or t-butylhydroperoxide. Thus, it would be beneficial to provide a homogeneousmixture of stable nanoparticles of cerium dioxide and water in anonpolar medium such as, for example, diesel fuel.

The invention provides a method for formulating a homogeneous mixturethat includes cerium dioxide nanoparticles, a nanoparticle stabilizer, asurfactant, water, and a nonpolar solvent. Preferably, the nanoparticleshave a mean diameter of less than about 10 nm, more preferably less thanabout 8 nm, most preferably about 6 nm.

As described above, cerium dioxide nanoparticles can be prepared byvarious procedures. Typical synthetic routes utilize water as a solventand yield an aqueous mixture of nanoparticles and one or more salts. Forexample, cerium dioxide particles can be prepared by reacting thehydrate of cerium(III)nitrate with hydroxide ion from, for example,aqueous ammonium hydroxide, thereby forming cerium(III)hydroxide, asshown in equation (2a). Cerium hydroxide can be oxidized tocerium(IV)dioxide with an oxidant such as hydrogen peroxide, as shown inequation (2b). The analogous tris hydroxide stoichiometry is shown inequations (3a) and (3b).

Ce(NO₃)₃(6H₂O)+2NH₄OH→Ce(OH)₂NO₃+2NH₄NO₃+6H₂O   (2a)

2Ce(OH)₂NO₃+H₂O₂→2CeO₂+2HNO₃2H₂O   (2b)

Ce(NO₃)₃(6H₂O)+3NH₄OH→Ce(OH)₃+3NH₄NO₃+6H₂O   (3a)

2Ce(OH)₃+H₂O₂→2CeO₂+4H₂O   (3b)

Complexes formed with very high base levels, e.g., 5:1 OH:Ce, alsoprovide a route to cerium dioxide.

In some cases, especially where ammonium hydroxide is not present inexcess relative to cerous ion, the species Ce(OH)₂(NO₃) or(NH₄)₂Ce(NO₃)₅ may initially be present, subsequently undergoingoxidation to cerium dioxide.

The cerium dioxide particles are formed in an aqueous environment andcombined with one or more nanoparticle stabilizers. Desirably, thecerium dioxide nanoparticles are either formed in the presence of thestabilizer(s), or a stabilizer(s) is added shortly after theirformation. Useful nanoparticle stabilizers include alkoxysubstitutedcarboxylic acids, α-hydroxyl carboxylic acids, pyruvic acid, and smallorganic polycarboxylic acids. Examples of alkoxysubstituted carboxylicacids include 2-(methoxy)ethoxy acetic acid and2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEA). Examples of α-hydroxycarboxylic acids include lactic acid, gluconic acid and2-hydroxybutanoic acid. Polycarboxylic acids includeethylenediaminetetraacetic acid (EDTA), tartaric acid, and citric acid.In desirable embodiments, the nanoparticle stabilizer includes acompound of formula (Ia) or formula (Ib), as described above.

The reaction mixture includes, in addition to cerium dioxidenanoparticles, one or more salts, for example, ammonium nitrate andunreacted cerium nitrate. The stabilized particles can be separated fromthese materials and salts by washing with 18 Mohm water in anultrafiltration or diafiltration apparatus. Low ionic strength (<3mS/cm) is highly desirable for the formation and stabilization ofretained water in a micellar state. The washed, stabilized ceriumdioxide nanoparticles may be concentrated, if desired, using asemi-porous membrane, for example, to form an aqueous concentrate of thenanoparticles. The particles may be concentrated by other means as well,for example, by centrifugation.

In one preferred embodiment, the cerium dioxide nanoparticles areconcentrated by diafiltration. The diafiltration technique utilizesultrafiltration membranes, which can be used to completely remove,replace, or lower the concentration of salts in thenanoparticle-containing mixture. The process selectively utilizessemi-permeable (semi-porous) membrane filters to separate the componentsof the reaction mixture on the basis of their molecular size. Thus, asuitable ultrafiltration membrane would be sufficiently porous so as toretain the majority of the formed nanoparticles, while allowing smallermolecules such as salts and water to pass through the membrane. In thisway, the nanoparticles and the associated bound stabilizer can beconcentrated. The materials retained by the filter, including thestabilized nanoparticles, are referred to as the concentrate orretentate, the discarded salts and unreacted materials as the filtrate.

Pressure may be applied to the mixture to accelerate the rate at whichsmall molecules passes through the membrane (flow rate) and to speed theconcentration process. Other means of increasing the flow rate includeusing a large membrane having a high surface area, and increasing thepore size of the membrane, but without an unacceptable loss ofnanoparticles.

In one embodiment, the membrane is selected so that the average poresize of the membrane is about 30% or less, 20% or less, 10% or less, oreven 5% or less than that of the mean diameter of the nanoparticles.However, the pore diameter must be sufficient to allow passage of waterand salt molecules. For example, ammonium nitrate and unreacted ceriumnitrate should be completely or partially removed from the reactionmixture. In one preferred embodiment, the average membrane pore size issufficiently small to retain particles of 3 nm diameter or greater inthe retentate. This would correspond to a protein size of approximately3 kilodaltons.

Desirably, the concentrate includes stabilized nanoparticles and residuewater. In one embodiment, the concentration of cerium dioxidenanoparticles is preferably greater than about 0.5 molal, morepreferably greater than about 1.0 molal, even more preferably greaterthan about 2.0 molal.

Once the concentrate is formed, it is combined with one or moresurfactants and a nonpolar solvent to form a homogeneous mixture. Thesurfactant is chosen so that a reverse micelle consisting of an aqueous,stabilized cerium dioxide nanoparticles dispersed in a nonpolar mediumis formed. Reverse micellar solutions consisting of particles in anaqueous environment dispersed in a nonpolar solvent, have been describedpreviously in, for example, Ying, et al., in U.S. Pat. No. 6,869,584 andU.S. Patent Appl. Publ. No. 2005/0152832, the disclosures of which areincorporated herein by reference.

Depending upon the relative sizes of the cerium dioxide nanoparticlesand the reverse micelle particles, the former may be incorporated intothe structure of the latter to varying extents. In one embodiment, thestabilized cerium dioxide nanoparticles are added, with mixing, to asolution of the surfactant and a co-surfactant and a nonpolar solvent ata temperature in the range of about 25° C. to about 0° C. Suitablenonpolar solvents include, for example, hydrocarbons containing about 6to 20 carbon atoms, for example, pentane, heptane, octane, decane andtoluene, and hydrocarbon fuels such as gasoline, biodiesel, and dieselfuels.

Useful surfactants include nonylphenyl ethoxylates having the formula,C₉H₁₉C₆H₄(OCH₂CH₂)_(n)OH, wherein n is 4-6. Other surfactants thatcontain both an ether group and an alcohol group includes compounds offormula (Ic), in which R³ represents a substituted or unsubstitutedalkyl group, and m is an integer of 1-8.

R³—(OCH₂CH₂)_(m)—OH   (Ic)

In certain embodiments, carboxylate surfactants such as the salts ofstearic acid, palmitic acid, and oleic acid may be useful assurfactants.

Another type of useful surfactant is represented by formula (Ib),wherein each R² independently represents a substituted or unsubstitutedalkyl group or a substituted or unsubstituted aromatic group, X and Zindependently represent H or a counterion such as Na⁺, or K⁺, and p is 1or 2.

XO₂C(CR²)_(p)CO₂Z   (Ib)

In another embodiment, the reverse-micelle forming agent includes ananionic surfactant and a nonionic co-surfactant. Useful co-surfactantsinclude aliphatic alcohols, for example, pentanol and hexanol and theirgeometric isomers.

Formulating cerium dioxide nanoparticle dispersions using a reversemicelle formation allows the aqueous nanoparticle stabilizing agent(s)to be independently optimized from that of the surfactant(s).

A desirable reverse-micellar composition is effective for lowering thecold pour cloud point of diesel fuel, that is, the temperature at whichwax crystals begin to form and the diesel fuel begins to gel. For adiscussion of the cold pour cloud point, see Langer et al., U.S. Pat.No. 6,368,366 and U.S. Pat. No. 6,383,237, the disclosures of which areincorporated herein by reference.

A desirable reverse-micellar composition is extremely stable and capableof very high dilution ratios; a dilution of 500:1 fuel:micellarcomposition or greater is, highly advantageous. To optimize thestability of the reverse-micellar composition, the cerium dioxidenanoparticle concentrate preferably includes high resistivity water,that is, water having a resistivity of about 1-18 mega ohm per cm,preferably about 18 mega ohm per cm. Pure water has a resistivity of18.3 mega ohm per cm.

Resistivity is the reciprocal of conductivity, which is the ability of amaterial to conduct electric current. Conductivity instruments canmeasure conductivity by including two plates that are placed in thesample, applying a potential across the plates (normally a sine wavevoltage), and measuring the current. Conductivity (G), the inverse ofresistivity (R), is determined from the voltage and current valuesaccording to Ohm's law, G=1/R=I/E, where I is the current in amps and Eis the voltage in volts. Since the charge on ions in solutionfacilitates the conductance of electrical current, the conductivity of asolution is proportional to its ion concentration. The basic unit ofconductivity is the siemens (S), or milli-Siemens (mS). Since cellgeometry affects conductivity values, standardized measurements areexpressed in specific conductivity units (mS/cm) to compensate forvariations in electrode dimensions.

In an optimal micellar composition, it is desirable that very few ionsbe present in the cerium dioxide concentrate to conduct electricity.This situation can be achieved by concentrating the cerium dioxideparticles through diafiltration to a conductivity level of less than 5mS/cm, preferably to 3 mS/cm or less.

The present invention is further directed to a method for formulating ahomogeneous mixture including cerium dioxide nanopartidles, at least onenanoparticle stabilizer and at least one surfactant, water, and anonpolar solvent. A first step provides an aqueous mixture includingstabilized cerium dioxide nanoparticles, wherein molecules of thenanoparticle stabilizer are closely associated with the nanoparticles. Asecond step includes concentrating the stabilized cerium dioxidenanoparticles while minimizing the ionic strength of the suspension toform an aqueous concentrate that is relatively free of anions andcations. A third step includes combining the concentrate with a nonpolarsolvent, containing a surfactant, thereby forming a substantiallyhomogeneous mixture that is a thermodynamically stable, multicomponent,single phase, reverse (“water in oil”) micellar solution.

The substantially homogeneous mixture contains water at a level ofpreferably about 0.5 wt. % to about 20 wt. %, more preferably, about5wt. % to about 15 wt. %. The cerium dioxide nanoparticles have a meanhydrodynamic diameter of preferably less than about 10 nm, morepreferably less than about 8 nm, most preferably about 6 nm. Desirably,the cerium dioxide nanoparticles have a primary crystallite size ofabout 2.5 nm±0.5 nm and comprise one or at most two crystallites perparticle edge length.

The aqueous mixture is advantageously formed in a colloid mill reactor,and the nanoparticle stabilizer may comprise an ionic surfactant,preferably a compound that includes a carboxylic acid group and an ethergroup. The nanoparticle stabilizer may comprise a surfactant of formula(Ia),

R—O—(CH₂CH₂O)_(n)CHR¹CO₂Y   (Ia)

wherein:

R represents hydrogen or a substituted or unsubstituted alkyl group or asubstituted or unsubstituted aromatic group;

R¹ represents hydrogen or an alkyl group;

Y represents H or a counterion; and

n is 0-5.

Preferably, R represents a substituted or unsubstituted alkyl group, R¹represents hydrogen, Y represents hydrogen, and n is 2.

Another suitable nanoparticle stabilizer comprises a compound of formula(Ib),

XO₂C(CR²)_(p)CO₂Z   (Ib)

wherein:

each R² independently represents hydrogen, a substituted orunsubstituted alkyl group or a substituted or unsubstituted aromaticgroup;

X and Z independently represent H or a counterion; and

p is 1 or 2.

Other useful nanoparticle stabilizers are included in the groupconsisting of lactic acid, gluconic acid enantiomers, EDTA, tartaricacid, citric acid, and combinations thereof.

The surfactant may also comprise a nonionic surfactant, preferably acompound comprising an alcohol group and an ether group, in particular,a compound of formula (Ic),

R³—(OCH₂CH₂)_(m)—OH   (Ic)

wherein:

R³ represents a substituted or unsubstituted alkyl group; and m is aninteger from 1 to 8.

The nonionic surfactant may also comprise a compound of formula (Id),

R³-Φ-(OCH₂CH₂)_(m)—OH   (Id)

wherein:

-   -   R³ represents a substituted or unsubstituted alkyl group; and    -   Φ is an aromatic group    -   m is an integer from 4 to 6.

The surfactant may also comprise an anionic surfactant, preferably acompound containing a sulfonate group or a phosphonate group. A usefulanionic surfactant is sodium bis(2-ethyl-1-hexyl)sulfosuccinate (AOT).

The aqueous reaction mixture may further include a co-surfactant,preferably an alcohol.

Concentrating the aqueous mixture is preferably carried out usingdiafiltration, which results in the reduction in conductivity of saidconcentrated aqueous mixture to about 3 mS/cm or less.

The nonpolar solvent included in the substantially homogeneous solutionis advantageously selected from among hydrocarbons containing about 6-20carbon atoms, for example, octane, decane, toluene, diesel fuel,biodiesel, and mixtures thereof. When used as a fuel additive, one partof the homogeneous mixture is with at least about 100 parts of the fuel.

Further in accordance with the present invention is a method forpreparing cerium dioxide nanoparticles comprising a core and a shell,wherein the shell comprises a material selected from the groupconsisting of a transition metal, a lanthanide, a sulfur-containingcompound that may include a mercaptide group, and combinations thereof.Preferably, the core comprises about 90% or less of the nanoparticle byvolume, and the shell comprises about 5% or more of the nanoparticle byvolume. The shell comprises lattice sites, and up to about 30% of thelattice sites include a material selected from the group consisting of atransition metal, a lanthanide, a sulfur-containing compound, andcombinations thereof.

The transition metal is preferably selected from the group consisting ofFe, Mn, Cr, Ni, W, Co, V, Cu, Mo, and Zr, or from the lanthanide series,and combinations thereof. Desirably, the transition metal is capable ofbinding to iron. It is also desirable that the transition metal becapable of reacting with an oxide of sulfur. In a further embodiment,the transition metal is associated with at least one ligand thatcomprises sulfur.

A composition comprising aqueously suspended cerium dioxidenanoparticles that comprise a core and a shell, wherein the shellincludes at least one transition metal, may be subsequently solventshifted into a non polar medium in which the particles are essentiallywater free and are incorporable into a lubrication oil. Thenanoparticles in the oil act as an adjuvant to further reduce frictionof contacting moving engine parts.

It would be beneficial to form a ceramic oxide coating on the surface ofdiesel engine cylinders in situ. The potential benefits of the coatinginclude added protection of the engine from thermal stress; for example,CeO₂ melts at 2600° C., whereas cast iron, a common material used in themanufacture of diesel engines, melts at about 1200-1450° C. Even 5 nmceria particles have demonstrated the ability to protect steel fromoxidation for 24 hours at 1000° C., so the phenomenon of size dependentmelting would not be expected to lower the melting point of the ceriumdioxide nanoparticles of the invention below the combustion temperaturesencountered in the engine. See, for example, Patil et al., Journal ofNanoparticle Research, vol. 4, pp 433-438 (2002). An engine so protectedmay be able to operate at higher temperatures and compression ratios,resulting in greater thermodynamic efficiency. A diesel engine havingcylinder walls coated with cerium dioxide would be resistant to furtheroxidation (CeO₂ being already fully oxidized), thereby preventing theengine from “rusting.” This is important because certain additives usedto reduce carbon emissions or improve fuel economy such as, for example,the oxygenates MTBE, ethanol and other cetane improvers such asperoxides, also increase corrosion when introduced into the combustionchamber, which may result in the formation of rust and degradation ofthe engine lifetime and performance. The coating should not be so thickas to impede the cooling of the engine walls by the water recirculationcooling system. In one embodiment, the current invention provides ceriumdioxide nanoparticles having a mean hydrodynamic diameter of less thanabout 10 nm, preferably less than about 8 nm, more preferably 6 nm oreven less, that are useful as a fuel additive for diesel engines. Thesurfaces of the cerium dioxide nanoparticles may be modified tofacilitate their binding to an iron surface, and desirably would, whenincluded in a fuel additive composition, rapidly form a ceramic oxidecoating on the surface of diesel engine cylinders.

In one embodiment, a transition or lanthanide metal having a bindingaffinity for iron is incorporated onto the surface of the cerium dioxidenanoparticles. Examples of iron surfaces include those that exist inmany internal parts of engines. Suitable transition metals include Mn,Fe, Ni, Cr, W, Co, V, Cu, and Zr.

The transition or lanthanide metal ion, which is incorporated into thecerium dioxide nanoparticles by occupying a cerium ion lattice site inthe crystal, may be introduced as a dopant during the latter stages ofthe precipitation of cerium dioxide. The dopant can be added incombination with cerous ion, for example, in a single jet manner inwhich both cerous ion and transition metal ion are introduced togetherinto a reactor containing ammonium hydroxide. Alternatively, the dopantand cerous ion can be added together with the simultaneous addition ofhydroxide ion. The doped particles can also be formed in a double jetreaction of cerous ion with dissolved transition metal ion titratedagainst an ammonium hydroxide steam simultaneotsly introduced by asecond jet. In any event, it is understood that sufficient nanoparticlestabilizer is present to prevent agglomeration of the nascent particles.

In a further embodiment, cerium dioxide nanoparticles are preparedhaving a core-shell structure. The core of the particle preferablyincludes at least about 75% more preferably, about 95% or greater of thebulk particle, and may be optionally doped with a metal. The shell,including the outer portion and surface of the particle, preferablycomprises about 25% or less, more preferably about 10% or less, mostpreferably about 5% or less, of the particle, and includes a transitionor lanthanide metal. Up to about 30% of the Ce⁺⁴ lattice sites of theshell may occupied by one or more transition or lanthanide metals.Suitable transition metals include Mn, Fe, Ni, Cr, W, Co, V, Cu, Zr, andMo, and combinations thereof.

In a further embodiment, the cerium dioxide nanoparticles have acore-shell structure, wherein the shell includes at least one compoundcomprising sulfur. Preferably, the sulfur is present so that it iscapable of forming a bond with iron. When used as a fuel additive for adiesel engine, the sulfur contained in the shell of the cerium dioxideparticles binds to the iron surface of the combustion chamber of theengine, thereby accelerating the deposition of cerium dioxide on thesurface of the combustion chamber. Suitable sulfur compounds includeZnS, MnS, FeS, Fe₂S₃, CoS, NiS, and CuS. The sulfur may be part of atransition metal ligand, wherein the metal and its associated ligand areincorporated into the surface of the cerium dioxide nanoparticles. Forexample, ligands that include a mercaptide group can form sulfur-ironbonds.

Sulfur can be incorporated into the cerium dioxide nanoparticles duringthe aqueous precipitation of CeO₂, for example, by incorporating withthe cerium nitrate hexahydrate reactant the appropriate water solubletransition metal salt (nitrate, sulfate or chloride), together with alabile source of sulfur such as thiosulfate (alternatively, thethiosulfate salt of a transition metal may be used). During the thermalconversion of the cerium hydroxide to the oxide at elevatedtemperatures, for example, about 70-90° C., the corresponding transitionmetal sulfide will also form.

In another embodiment, a transition metal is incorporated into thesurface of the cerium dioxide nanoparticles. Desirably, this metal ischosen so that it is capable of reacting with sulfur and forming a bondto sulfur. The transition metal is present in the reaction mixtureduring the shell formation of the CeO₂ precursor (cerium hydroxide).Suitable metals include Mn and Fe as well as W, Co, V, Cu, and Mo.Typical aqueous soluble transition metal salts include sulfates,nitrates, and chlorides of these metals.

When used as a fuel additive, the transition metal-containingnanoparticles can bind sulfur that may be present in the fuel. Iron, forexample, can react with sulfur dioxide to form Fe₂S₃. This reduces thelevel of reactive sulfur, for example, sulfur oxides, present in gasesemitted from the fuel combustion chamber. Removal of sulfur after fuelcombustion is very desirable, since many vehicle exhaust systems includeparticulate traps containing a platinum catalyst that can be poisoned bysulfur. Hence removal of sulfur before it reaches the catalyst canprolong the life of the catalyst. Useful metals for the reduction ofsulfur dioxide are also described by Yamashita , et al., U.S. Pat. No.5,910,466, the disclosure of which is incorporated herein by reference.

It is known in the art that small particles can be made within theisolated phase of an emulsion, which is a stable mixture of at least twoimmiscible liquids. Although immiscible liquids tend to separate intotwo distinct phases, an emulsion can be stabilized by the addition of asurfactant that functions to reduce surface tension between the liquidphases. An emulsion comprises a continuous phase and a disperse phasethat is stabilized by a surfactant. A water-in-oil (w/o) emulsion havinga disperse aqueous phase and an organic continuous phase, typicallycomprising a hydrocarbon, is often referred to as a “reverse-micellarcomposition.”

Further in accordance with the invention, a reverse-micellar compositioncomprises a disperse phase comprising a cerium (IV)nanoparticle-containing aqueous composition, together with a continuousphase comprising a hydrocarbon liquid and at least one surfactant. Afuel additive composition of the inyention comprises a reverse-micellarcomposition whose aqueous disperse phase includes in situ-formednanoparticles comprising a cerium (IV) oxidic compound, and whosecontinuous phase includes a hydrocarbon liquid and asurfactant/stabilizer mixture. The surfactant/stabilizer mixture iseffective to restrict the size of the nanoparticles thus formed,preventing their agglomeration and enhancing the yield of thenanoparticles.

In another embodiment, a reverse-micellar composition comprises: anaqueous disperse phase that includes a free radical initiator, and acontinuous phase that includes a hydrocarbon liquid and at least onesurfactant. Optionally, the reverse-micellar composition may includecerium-containing nanoparticles.

In a further embodiment, a fuel additive composition comprises: acontinuous phase comprising a hydrocarbon liquid, a surfactant, andoptionally a cosurfactant; and forming a reverse-micellar compositioncomprising an aqueous disperse phase that includes a cetane improvereffective for improving engine power during combustion of the fuel. Thefuel additive composition optionally further comprises cerium-containingnanoparticles, which may be included in either a separate dispersion ora separate reverse-micellar composition.

In one embodiment of the present invention, a water-in-oil emulsion hasa small micellar disperse size, and the particulate material is formedwithin the aqueous disperse phase. The appropriate choice of surfactantsand reaction conditions provides for the formation of stable emulsions,the control of particle size distribution and growth, and the preventionof particle agglomeration. The oil phase preferably comprises ahydrocarbon, which may further include oxygen-containing compounds. Inthe micelle, the disperse aqueous phase is encompassed by a surfactantboundary that isolates and stabilizes the aqueous phase from the organiccontinuous phase.

A surfactant included in the emulsion preferably in the continuous phaseto stabilize the reverse micelles can be an ionic surfactant, anon-ionic surfactant, or a combination thereof. Suitable surfactantsinclude, for example, nonylphenyl ethoxylates, monoalkyl and dialkylcarboxylates, and combinations thereof.

The difficulties of using two distinct reverse micelles for thecerium-containing reactant and a precipitating agent such as ammoniumhydroxide are avoided by the present invention, which provides for thecombination of both reactants into a single reverse micelle using ahomogeneous precipitation method, wherein a first reactant ishomogeneously mixed with a precursor of a second reactant. A suitablefirst reactant is a Ce⁺⁴-containing compound, which may be obtained byoxidation using H₂O₂ for example, of a Ce⁺³-containing compound such as,for example, Ce(NO₃)₃.6H₂O.

A suitable second reactant is ammonia, NH₃, which can be obtained by theheat- and/or light activated hydrolysis of hexamethylenetetramine,C₆H₁₂N₄, (HMT), as shown in equation (4):

C₆H₁₂N₄+12H₂O→4NH₃+6CH₂O   (4)

The homogeneous precipitation of cerium dioxide using HMT has beenreported by Zhang, F., Chan Siu-Wai et al., Applied Physics, 80, 1(2002), pp127-129, and in the previously discussed Chan, U.S. PatentAppl. Publ. No. 2005/0031517. In the absence of a stabilizer, the sizeof the cerium dioxide particles produced by the procedure described inthese references continues to increase with time. Furthermore, theprocedure utilizes very dilute solutions and long reaction times, andproduces low product yield.

In an example of the process of the present invention, whichbeneficially combines reverse micelle with homogeneous precipitationtechniques, Ce(NO₃)₃(6H₂O) is combined with H₂O₂ to generate aCe⁺⁴-containing solution. Preferably, the solution further includes astabilizer for controlling the size of the cerium-containingnanoparticles. A preferred stabilizer is2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEA). The resulting solutionis added to a cold HMT solution at a temperature sufficiently low, lessthan about 15° C., to inhibit premature reaction. The resulting mixtureis then slowly added to an oil phase comprising a surfactant and anorganic solvent such as, for example, toluene, octane, decane, gasoline,D2 diesel fuel, ULSD, biodiesel, or combinations thereof. The newmixture is heated to a temperature just sufficient to effectsubstantially complete formation of the Ce-containing nanoparticles. Theprecise temperature required depends on the choice of reverse-micellesurfactant and the concentration of the first reactant and secondreactant precursor but is desirably maintained below about 47° C. Thereverse-micelle surfactant may also serve to stabilize the Ce-containingnanoparticles. Alternatively, the aqueous Ce⁺⁴-HMT mixture may bepremixed with another surfactant different from that used to form thereverse-micellar composition. The aqueous composition may optionallyfurther include a cetane improving agent generally recognized to be afree radical forming species at elevated temperatures.

Depending on the reaction conditions, the individual micelles may besmall enough to encompass a single cerium-containing nanoparticle orlarge enough to contain a plurality of the nanoparticles. Thus, themicelles have a diameter of preferably about 5 nm to about 50 nm, morepreferably about 20 nm. The cerium-containing nanoparticles have adiameter of preferably about 1 nm to about 15 nm, more preferably about2 nm to about 10 nm.

The CH₂O generated in the aqueous phase by the hydrolysis of HMT may beutilized in a subsequent fuel combustion process. Alternatively, if thereverse micelle contains some cross-linkable groups, the CH₂O can effectcross-linking within the micelle, strengthening it or increasing itsheat-resistance.

A fuel additive emulsion formed by the reverse micelle process of thepresent invention includes water used in the preparation of thecerium-containing nanoparticles. Excess water introduced into a fuelwith the cerium-containing emulsion can lead to a loss of engine power.To overcome this problem and thereby improve fuel performance, water canbe removed from the cerium-containing aqueous phase and replaced by acetane improver. Water removed by, for example, diafiltration may bereplaced by a water-soluble cetane improving compound. Compoundssuitable for this purpose include, for example, 30-50 wt. % aqueousH₂O₂, t-butyl hydroperoxide, nitromethane, and low molecular weightalkyl ethers such as dimethyl ether and diethyl ether.

Free radical initiators such as, for example, H₂O₂ are known to beeffective cetane improvers for diesel fuel, resulting in reductions insoot and hydrocarbon emission. Cetane number is an indicator of theignition delay time after injection of fuel into the combustion chamber;alternatively, it can be regarded as being related to the inverse of theignition time, i.e., the time between the injection of the diesel fuelinto the compressed superheated air in the combustion chamber and theactual ignition of the injected fuel stream. The higher the cetanenumber, the more completely combusted the fuel and the less sootproduction, as ignition delay gives rise to the formation of soot. Anadditional consideration is the desire for this ignition to occur asclosely as possible in time to when the piston reaches top-dead-center(TDC), since too short an ignition time would result in the combustedgases working against the compressive stroke of the piston. For 12-literdiesel and smaller engines, fuel injection usually occurs at a crankangle of 5 or 6 degrees before TDC. Thus, cetane improvement would havea very small effect on the crank angle and minimal adverse effect onengine power. On the other hand, substantial cetane improvement withdiesel locomotive engines, which have a 25 degree crank angle, would beproblematic for engine power without prior adjustment of the crankangle.

Utilization of a free radical mechanism for enhanced combustionefficiency is a very attractive alternative to simply increasing the O₂stoichiometry in the combustion chamber, since free radical chemistryinvolving O atoms or OH species is roughly two orders of magnitudefaster than direct oxidation by O₂, as represented in equation (5):

C₁₄H₃₀+22O₂→15H₂O+14CO₂   (5)

This is partly a consequence of the need to initially rupture a O═O bond(bond dissociation energy delta H of 119.2 Kcal/mole) and the highreactivity of OH radicals, which are one of the most chemically reactivespecies that can be generated (on an electromotive force scale or freeenergy scale), just slightly less reactive than fluorine radicals.

Oxidation of hydrocarbons and soot by free radical chemical chemistry,on the other hand, can involve breaking a relatively weak O—O singlebond (delta H=47 kcal/mole for hydrogen peroxide) and then proceed viadirect C—H bond scission to give water and a “hot,” i.e., chemicallyreactive, hydrocarbon radical, as shown in equation (7):

H₂O₂→2.OH   (6)

H₃C—C₁₃H₂₇+.OH→H₂O+.CH₂—C₁₃H₂₇   (7)

H₂O₂→H₂O+½O₂   (8)

This highly reactive hydrocarbon radical can subsequently readilyundergo oxidation. According to Born and Peters in “Reduction of SootEmission in a DI Diesel Engine of Hydrogen Peroxide during Combustion,”S.A.E. Technical Paper 982676 (1998), equation (7) represents thedominant reaction path for the decomposition of peroxide at temperaturesabove 727° C., not the thermolytic reaction generating water and oxygen,as shown in equation (8).

Maganas et al., U.S. Pat. No. 6,962,681, the disclosure of which isincorporated herein by reference, describes a system whereincatalytically reactive particles of silica or alumina interact with themoisture in combustion exhaust gases to generate hydroxyl radicals,which are returned to the site of combustion and increases theefficiency of combustion, resulting in reduced soot formation.

Hashimoto et al., U.S. Patent Application Serial No. 2006/0185644, thedisclosure of which is incorporated herein by reference, describes afuel composition that includes 95-99.5 wt. % of a base fuel and 0.1-5wt. % of an additive compound selected from the group consisting of anorganic peroxide such as di-t-butyl peroxide, a nitrate ester such asn-pentyl nitrate, a nitrite ester such as n-pentyl nitrite, and an azocompound such as 2,2-azobis(2,4-dimethylvaleronitrile).

The inclusion of a free radical initiator in a fuel additive compositionof the present invention provides multiple advantages:

When incorporated in a separate reverse-micellar composition orco-incorporated with a CeO₂ fuel borne additive in a reverse micelle, itprovides a mechanism by which the internal engine components are“cleaned” or scrubbed of residual soot, thereby providing a freshsurface. This greatly accelerates the rate at which the cerium dioxidenanoparticles can be incorporated into the cast iron matrix of theengine, thereby reducing the time it takes to “condition” the engine,i.e., provide it with a coating of catalytic nanoparticles that resultsin an increase in mpg economy. Additionally, the preferred stabilizersfor CeO₂ nanoparticles, for example, hydroxycarboxylic acids such aslactic and gluconic acids, are themselves potent free radical generatorsat high temperatures.

Even in a fully conditioned diesel engine in which the interior surfacesare rendered into a ceramic catalyst, the free radical mechanism wouldstill account for most of the observed increase in fuel efficiency,owing to the fact that only 25% of the injected fuel actually comes incontact with the cylinder walls and thus becomes available for catalyticcombustion; the majority of the fuel being combusted in the space overthe piston head. Thus a fuel-borne additive that contains awater-soluble free radical initiator such as H₂O₂ within a reversemicelle would be very useful.

Additionally, a fuel-borne additive in which the reverse micellecontains only a free radical precursor could be used to great advantagewith a nanoparticulate lubricity enhancing agent introduced as acomponent of the lubrication oil.

Generally, reverse micellar compositions having very small disperseparticle diameters, preferably about 5 nm to about 50 nm, morepreferably about 10 nm to about 30 nm, are very effective, as theirdisintegration and attendant release of superheated steam helps to mixthe additive-containing diesel fuel with air in the combustion chamber,resulting in more complete fuel combustion.

Preferably, the free radical initiators included in the reverse micellein accordance with the present invention have substantialwater-solubility. The following patents, the disclosures of which areincorporated herein by reference, teach the use of water-soluble freeradical initiators:

U.S. Pat. No. 3,951,934 discloses azo-bis compounds, as well ascombinations of water-soluble peroxides with tertiary amines, sulfites,and bromates.

U.S. Pat. No. 5,248,744 teaches azo-bis compounds as well asperoxydisulfates and organic peroxides.

U.S. Pat. No. 6,391,995 discloses the use of water-soluble azoinitiators, including four compounds commercially available from WakoChemicals, Dallas Tex.

Oak Ridge National Laboratory document TM-11248 by W. V. Griffith and A.L. Compere includes an extensive list of cetane improvers for increasingengine power that may be included in the reverse-micellar compositionsof the present invention. Useful compounds for this purpose includealkyl nitrates, esters, azoles, azides, ethers, and hydroperoxides suchas cumene hydroperoxide.

Puchin et al., USSR patents 236,987 and 214,710 (1970), discloses thatpoly(dimethyl(vinylethynyl)methyl) t-butyl peroxide at a 0.01% level,i.e. 100 ppm, gives a Δ cetane % additive ratio of 1000, correspondingto a cetane improvement of 10. The references also disclose “otheradditives” that may be small mono esters incorporated into aqueousmicelles, or even long chain fatty acid mono esters (high cetane rating)that would not require incorporation as a reverse micelle but might actas a surfactant for a reverse micelle emulsion.

Hicks et al., U.S. Patent Appl. Publ. No. 2002/0095859, the disclosureof which is incorporated herein by reference, states that highsurfactant to water ratios on the order of 2.5:1 in a concentratedmicro-emulsion forming fuel additive produces improved hydrocarbon fuelcombustion at only 5 to 95 ppm of additional water.

A fuel additive composition of the present invention may comprise morethan one type of reverse micelle. For example, one type of reversemicelle may include a cetane improver, and a second type reverse micellemay include cerium-containing nanoparticles together with associatedreverse micellar phase water that may be at least partially replaced bya free radical initiator such as hydrogen peroxide or, more preferably,a stabilized hydrogen peroxide.

In accordance with the present invention, a cerium-containing fueladditive composition includes a surfactant/stabilizer mixture thatpreferably includes a combination of at least one non-ionic surfactantwith at leak one anionic surfactant, or a combination of asingle-charged anionic surfactant and a multiple-charged anionicsurfactant. The effect of the combination of surfactant/stabilizercompounds is to restrain the size of the nanoparticles, prevent theiragglomeration, and enable an increase in the concentration of reactants,thereby producing a higher yield of nanoparticles.

The surfactant/stabilizer combination may have the added benefit ofaiding in the solvent shift process from the aqueous polar medium to thenon-polar oil medium. In a combination of charged and unchargedsurfactants, the charged surfactant compound plays a dominant role inthe aqueous environment. However, as solvent shifting occurs, thecharged compound is likely to be solubilized into the aqueous phase andwashed out, and the uncharged compound becomes more important instabilizing the reverse micelle emulsion.

Dicarboxylic acids and their derivatives, so called “geminicarboxylates”, where the carboxylic groups are separated by at most twomethylene groups, are also useful cerium dioxide nanoparticlestabilizers. Additionally, C₂-C₈ alkyl, alkoxy and polyalkoxysubstituted dicarboxylic acids are advantageous stabilizers.

In accordance with the invention, nanoparticle stabilizer compoundspreferably comprise organic carboxylic acids such as, for example,2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEA) andethylenediaminetetraacetic acid (EDTA), lactic acid, gluconic acid,tartaric acid, citric acid, and mixtures thereof.

A reverse-micellar composition in accordance with the present inventioncomprises an aqueous disperse phase that includes a free radicalinitiator, preferably water-soluble, and a continuous phase thatincludes a surfactant, an optional co-surfactant, and a hydrocarbonliquid, preferably selected from among toluene, octane, decane, D2diesel fuel, ULSD, biodiesel, and mixtures thereof In general,hydrocarbons containing about 6-20 carbon atoms are useful. The aqueousdisperse phase of the composition comprises micelles having a meandiameter of preferably about 5 nm to about 50 nm, more preferably about3 nm to about 10 nm.

Free radical initiators suitable for inclusion in the aqueous dispersedphase may be selected from the group consisting of: hydrogen peroxide,organic hydroperoxides, organic peroxides, organic peracids, organicperesters, organic nitrates, organic nitrites, azobis compounds,persulfate compounds, peroxydisulfate compounds, and mixtures thereof.Preferred azobis compounds are selected from the group consisting of2-2′-azobis(2-methylpropionamidine) dihydrochloride;4-4′-azobis(4-cyanovaleric) acid;2-2′azobis[2-methyl-N-(2-hydroxyethyl)propionamide];2-2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, and mixturesthereof.

In a preferred embodiment, the free radical initiator in the aqueousdispersed phase comprises stabilized hydrogen peroxide or t-butylhydroperoxide. The aqueous disperse phase may further comprise, inaddition to the aforementioned peroxides, a compound selected from thegroup consisting of a tertiary amine compound, a sulfite compound, abromate compound, and mixtures thereof.

The reverse-micellar composition may further comprise boric acid or aborate salt in the aqueous disperse phase, and the hydrocarbon liquidpreferably comprises diesel fuel. In a further embodiment of theinvention, a lubricating oil that optionally contains cerium-containingnanoparticles may be used in conjunction with a fuel containing thereverse-micelle fuel additive.

The reverse micellar composition of the invention preferably includes asa radical initiator stabilized hydrogen peroxide or t-butylhydroperoxide in the aqueous phase at a level of 30%, 40%, or even 50%or greater by weight. In another embodiment, within the reverse micellarcomposition the ratio of water to hydrocarbon by weight is greater thanor equal to about 5%, about 10%, or preferably, greater than or equal toabout 15% by weight. In a further embodiment, the reverse micellarcomposition includes an alcohol such as hexanol, and/or an alkoxylatesurfactant such as Triton N-57.

A method for improving the performance of a diesel engine includesadding to diesel fuel, for example, D2 diesel or biodiesel, a reversemicellar composition comprising an aqueous first disperse phase thatincludes a free radical initiator and a first continuous phase thatincludes a first hydrocarbon liquid and at least one first surfactant.Suitable free radical initiators such as hydrogen peroxide or t-butylhydroperoxide, suitable hydrocarbon solvents preferably containing about6 to about 20 carbon atoms, and suitable surfactants were describedabove. Preferred surfactants include only the elements C, H, and O.Preferably the aqueous disperse phase includes about 20 wt. %, or 30 wt.%, or more preferably 40 wt. % or more of the radical initiator.Operating the engine and combusting the modified diesel fuel providesimproved engine efficiency relative to unmodified diesel fuel.Preferably, the modified diesel fuel includes less than 500 ppm waterunless accompanied by an equal amount of free radical initiator.

A useful reverse micellar composition for use as a diesel fuel additiveincludes an aqueous disperse phase that includes a boric acid or aborate salt, and a continuous phase that includes a surfactant and ahydrocarbon liquid. Examples of useful borate salts include, forexample, sodium borate and potassium borate. Examples of usefulhydrocarbon liquids include toluene, octane, decane, D2 diesel fuel,biodiesel, and mixtures thereof. In general, hydrocarbons containingabout 6-20 carbon atoms are useful. Suitable surfactants include AerosolAOT; however, as already mentioned, preferred surfactants include onlythe elements C, H, and O. Desirably, the aqueous disperse phase of thecomposition comprises micelles having a mean diameter of, preferably,about 5 nm to about 50 nm, more preferably, about 10 nm to about 30 nm.

A method for improving diesel engine performance includes the additionof an additive as described above to diesel fuel to obtain modifieddiesel fuel. Such an additive, when used in combination with dieselfuel, may provide improved diesel fuel mileage, reduced diesel enginewear, or reduced pollution or a combination of these features.

Motor oil is used as a lubricant in various kinds of internal combustionengines in automobiles and other vehicles, boats, lawn mowers, trains,airplanes, etc. In engines there are contacting parts that move againsteach other at high speeds, often for prolonged periods of time. Suchrubbing motion causes friction, forming a temporary weld, which absorbsotherwise useful power produced by the motor and converting the energyto useless heat. Friction also wears away the contacting surfates ofthose parts, which may lead to increased fuel consumption and lowerefficiency and degradation of the motor. In one aspect of the invention,a motor oil includes a lubricating oil, cerium dioxide nanoparticles,desirably having a mean diameter of less than about 10 nm morepreferably 5 nm , and optionally, a surface adsorbed stabilizing agent.

Diesel lubricating oil is essentially free of water, preferably lessthan 300 ppm, but may be desirably modified by the addition of a ceriumdioxide-containing reverse-micellar composition in which the ceriumdioxide has been solvent shifted from its aqueous environment to that ofan organic or non-polar environment. The cerium dioxide compositionsinclude nanoparticles having a mean diameter of less than aboutl 0 nmmore preferably about 6 nm, as already described. A diesel engineoperated with modified diesel fuel and modified lubricating oil providesgreater efficiency and may, in particular, provide improved fuelmileage, reduced engine wear or reduced pollution, or a combination ofthese features.

Metal polishing, also termed buffing, is the process of smoothing metalsand alloys and polishing to a bright, smooth mirror-like finish. Metalpolishing is often used to enhance cars, motorbikes, antiques, etc. Manymedical instruments are also polished to prevent contamination in marksin the metals. Polishing agents are also used to polish optical elementssuch as lenses and mirrors to a surface smoothness within a fraction ofthe wavelength of the light they are to manage. Smooth, round, uniformcerium dioxide particles of the present invention may be advantageouslyemployed as polishing agents, and may further be used for planarization(rendering the surface smooth at the atomic level) of semiconductorsubstrates for subsequent processing of integrated circuits.

Nanoparticles, or quantum dots, are being considered for many potentialapplications. Because of their small size, on the order of 1- 20 nm,these nanoparticles have properties different from their bulk versions,100 nm and larger. They exhibit novel electronic, magnetic, optical,chemical, and mechanical properties that make them attractive for manytechnological applications. Those nanoparticles that fall into thesemiconductor material category are being considered for biologicallabeling and diagnostics, light emitting diodes, solid-state lighting,photovoltaic devices, and lasers. Cerium dioxide nanoparticles arewide-gap semiconductors that are potentially useful in suchapplications. Furthermore, suitably doped versions of cerium dioxidenanoparticles could extend the range of applications.

There are two critical properties of nanoparticulate ceria that make ituniquely suited for medical applications.

First and perhaps most critically, is ceria's very low to non existenttoxicity to humans, a conclusion based upon human cell culture and otherdata, (Evaluation of Human Health Risk from Cerium Added to Diesel Fuel:Communication 9, 2001 Health Effects Institute, Boston Mass. andDevelopment of Reference Concentrations for Lanthanide, ToxicologyExcellence for Risk Assessment, The bureau of Land Management, NationalApplied Resource Sciences Center, Amended Stage 2, November 1999).

The second property involves the utility of the Ce³⁺/Ce⁴⁺ redox couple.Reactive free radical species such as the hydroxyl radical (OH) that cancause cellular damage in the body can be chemically reduced to therelatively harmless hydroxyl anion (OH⁻) by Ce³⁺. Conversely, anothercellular damaging radical species, the oxygen radical anion (O₂.⁻) canbe oxidized to molecular oxygen by Ce⁴⁺:

There have appeared a number of reports that describe the exploitationof these properties of nanoparticulate ceria, for example, to preventretinal damage induced by intracellular peroxides (Chen, et. al. NatureNanotechnology, 1, p 142, November 2006) and tumor studies in whichceria confers radioprotection upon healthy but not cancerous cells(Tarnuzzer, et. al., NanoLetters 5, 12, p 2573, 2005).

Suitably engineered nanoparticulate ceria, along with othernanomaterials, may be used as a biotag exploiting surface enhanced Ramanspectroscopy for fields such as immunodiagnostics, molecular diagnosticsand proteomics.

The invention is further illustrated by the following examples. Theseexamples are not intended to limit the invention in any manner.

Example 1 Preparation of Cerium Dioxide Nanoparticles: Single JetAddition

To a 3 liter round bottom stainless steel reactor vessel was added 1.267liters of distilled water, followed by 100 ml of Ce(NO₃)₃.6H₂O solution(600 gm/liter Ce(NO₃)₃.6H₂O). The solution was clear and has a pH of 4.2at 20° C. Subsequently, 30.5 gm of 2-[2-(2-methoxyethoxy)ethoxy]aceticacid (MEEA) was added to the reactor vessel. The solution remainedclear, and the pH was 2.8 at 20° C. A high sheer mixer was lowered intothe reactor vessel, and the mixer head was positioned slightly above thebottom of the reactor vessel. The mixer was a colloid mill manufacturedby Silverson Machines, Inc., modified to enable reactants to beintroduced directly into the mixer blades by way of a peristaltic tubingpump. The mixer was set to 5,000 rpm, and 8.0 gm of 30% H₂O₂ was addedto the reactor vessel. Then 16 ml of 28%-30% NH₄OH, diluted to 40 ml,was pumped into the reactor vessel by way of the mixer head in about 12seconds. The initially clear solution turned an orange/brown in color.The reactor vessel was moved to a temperature controlled water jacket,and a mixer with an R-100 propeller was used to stir the solution at 450rpm. The pH was 3.9 at 25° C. at 3 minutes after pumping the NH₄OH intothe reactor. The temperature of the reactor vessel was raised to 70° C.over the next 25 minutes, at which time the pH was 3.9. The solutiontemperature was held at 70° C. for 20 minutes, during which time thesolution color changed from orange brown to a clear dark yellow. The pHwas 3.6 at 70° C. The temperature was lowered to 25° C. over the next 25minutes, at which time, the pH was 4.2 at 25° C. Particle size analysisby dynamic light scattering indicateda cerium dioxide intensity weightedhydrodynamic diameter of 6 nm. The dispersion was then diafiltered to aconductivity of 3 mS/cm and concentrated, by a factor of about 10, to anominal 1 Molar in CeO₂ particles.

The cerium dioxide particles were collected, the excess solventevaporated off, and the gravimetric yield, corrected for the weight ofMEEA, was determined to be 26%. The size distribution of the ceriumdioxide particles (plotted in FIG. 4), determined by dynamic lightscattering, indicated a particle size having a mean intensity weightedhydrodynamic diameter of about 6 nm. Over two dozen replicatedprecipitations and independent measurements of these precipitations gavea mean intensity weighted size of 5.8 nm±0.4 nm (one standarddeviation). Thus, the reaction precipitation scheme is robust.Additionally, the size distribution is substantially monomodal, i.e.,only one maximum, with most of the particles falling in the range 5.2 nmto 6.4 nm. Feature 55 of the size distribution is a binning artifact.

A transmission electron microscope (TEM) was also used to analyze thecerium dioxide particles. A 9 microliter solution (0.26M) was dried ontoa grid and imaged to produce the image 60, shown in FIG. 5. The darkcircular features 61 are the imaged particles. The particles show nosigns of agglomeration, even in this dried-down state. In solution, theparticles would be expected to show even less propensity to agglomerate.The gradicule (61) represents 20 nm; it is clear from FIG. 5 that themean particle size is quite small, less than 10 nm. From severalmicrographs such as these, particles were individually sized and themean was calculated to be 6.7±1.6 nm. This independently corroboratesthe sizing data measured by dynamic light scattering.

FIG. 6 shows an X-Ray powder diffraction pattern 70 of a sample of thedried cerium dioxide nanoparticles, together with a reference spectrum71 of cerium dioxide, provided by the NIST (National Institute ofStandards and Technology) library. The line positions in the samplespectrum match those of the standard spectrum. The two theta peak widthswere very wide in the sample spectrum, which is consistent with a verysmall primary crystallite size and particle size. From the X-Ray data(Cu K alpha line at about 8047 ev) and the Scherrer formula(d=0.9*lambda/delta*cos(theta), where lambda is the x-ray wavelength,delta the full width half maximum, and theta the scattering anglecorresponding to the x ray peak), the primary crystallite size iscalculated to be 2.5±0.5 nm (95% confidence of 5 replicas)

Examples 1a-f Evaluation of Alternative Stabilizers to MEEA

Example 1 was repeated, except that in Example 1a an equivalent molaramount of succinic acid was substituted for the MEEA stabilizer. A brownprecipitate that readily settled was obtained, which is an indication ofvery large particles (several tens of microns). The same experiment wasrepeated each time substituting an alternative stabilizer (malonicacid-Example 1b, glycerol-Example 1c, ethyl acetoacetate-Example 1d). Ineach case, a readily settling brown precipitate was obtained, indicatingthe failure to obtain nanoparticles. For Example 1e, lactic acid attwice the molar concentration was substituted for the MEEA stabilizer.Quasi-inelastic dynamic light scattering measurements revealed a meanhydrodynamic diameter particle size of 5.4 nm when the hydroxide wasdoubled, and 5.7 nm when the hydroxide was increased by 75%. Mixtures ofEDTA (which by itself produces no particles) and lactic acid at a ratioof about 20%/80% also gave particles of CeO₂ with a hydrodynamicdiameter of 6 nm. In Example 1f, the optimal EDTA: lactic acid ratio of1:4 was used, but at twice the overall concentration of this stabilizermixture, which resulted in a decrease in the mean particle size to 3.3nm. At a three times level (same ratio) there were no particles formed(the stabilizer effectively complexed all the free cerium ion,preventing the formation of the hydroxide). It is therefore possible tocontrol the particle size by adjusting the stabilizer component ratiosand overall stabilizer concentration levels.

Example 2 CeO₂ Precipitation with EDTA/Lactic Acid Stabilizer-Effect ofMixing

To a 3 liter round bottom stainless steel reactor vessel was added 76.44gm EDTA disodium salt in distilled water to a total weight of 1000gm,74.04 gm of DL-lactic Acid (85%), 240.0 gm of Ce(NO₃)₃.6H₂O in 220 gm ofdistilled water and 19.2 gm of 50% H₂O₂ aqueous solution. As in Example1, the mixer speed was set to 5000 rpm, and the contents of the reactorwere brought to a temperature of 22° C. Separately, a solution of 128.0gm NH₄OH (28-30%) was prepared. This quantity of hydroxide is equivalentto twice the number of moles of cerium solution, so the initiallynucleated precipitate was presumably the bis-hydroxyl intermediate. Inone experiment, the ammonium hydroxide solution was single jetted intothe reactor in the reaction zone defined by the mixer blades andperforated screen. In another experiment, the hydroxide was added via asingle jet just subsurface into the reactor in a position remote fromthe active mixing zone of the colloid mixer. After the usual heattreatment and filtration, the intensity weighted diameter of the CeO₂particles produced at the actively mixed zone was 6.1 nm, with apolydispersity of 0.129. The diameter of the particles produced via thesecond method, i.e., sub-surface introduction of the of the ammoniumhydroxide at a position remote from the reaction zone, was essentiallythe same, 6.2 nm, but the polydispersity was much greater, 0.149. Thus,the size frequency distribution can be narrowed by mixing in the highshear region of the colloid mill.

Example 3 CeO₂ Particle Size Dependence Upon Hydroxide Stoichiometry

The conditions of this experiment follow that of Example 2, except thatthe cerium ion was not in the reactor but was separately introduced viaa jet into the reaction zone simultaneously with the jetting of theammonium hydroxide solution. Three molar stoichiometric ratios ofhydroxide ion to cerium ion were explored: 2:1, 3:1 and 5:1. Thefollowing table summarizes the intensity weighted particle sizediameters and polydispersities obtained by the quasi-inelastic dynamiclight scattering technique.

Gravimetric Yield OH:Ce CeO₂ CeO₂ (1000° C. muffle mole ratio diameter(nm) Polydispersity furnace) 2:1 5.8 0.110 51.7% 3:1 10.2 0.158 57.2%5:1 12.5 0.156 49.9%

It is clear from the data that the smallest, most uniformly distributedparticles can be obtained in good yield by this double jet procedurewhen the molar ratio of hydroxide to cerium is 2:1. The size of theparticles obtained in higher yield under 3:1 stoichiometry conditionsmay be reduced by a suitable increase in the stabilizer level, as wasdemonstrated in Example 1f.

Example 4 CeO₂ Precipitation Temperature Effects

The effect of low temperature nucleation at 20° C., followed hydroxideconversion to the oxide at 70° C., versus an isothermal precipitation inwhich both nucleation and conversion were conducted at 70° C. wasinvestigated using the reagent conditions specified in Example 2. Thepreferred double jet method was employed (separate jets for cerium ionand hydroxide ion, both introduced into the reactive mixing zone of thecolloid mixer). The ammonium hydroxide concentration was at the 128 gm,i.e., 2× level or a OH:Ce molar stoichiometric ratio of 2:1.Quasi-inelastic dynamic light scattering measurements revealed that theparticles made at the lower temperature precipitation had an intensityweighted hydrodynamic diameter of 5.8 nm, with a polydispersity of0.117, and a yield of 54.6%, while the isothermal precipitation gavelarger particles, 8.1 nm, that were more widely distributed, with apolydispersity of 0.143, in comparable yield. Thus, if a more uniformparticle size frequency distribution is desired, it is preferable tonucleate at lower temperature before carrying out the higher temperatureconversion of the hydroxide to the oxide.

Example 5 Preparation of Cerium Oxide-Containing Additive Formulationsof Varying Batch Size

Formulations with volumes of 207 ml, 1.5 liters, and 9.5 liters wereprepared according to procedures summarized in the following table:

Approximate Batch Volume 207 ml 1.5 liters 9.5 liters Reactor 250-mlS.S. 3-liter S.S. 11-liter S.S. beaker w/magnetic round-bottomedround-bottomed stirring bar vessel vessel Solution PreparationComponents Distilled water in reactor 127 g 1.267 kg 8.2355 litersCe(NO₃)₃•6H₂O 8.52 g in distilled 60 g in H₂O 390 g in H₂O H₂O to 25 mlto 100 ml to 500 ml Stabilizer - MEEA 4.36 g in distilled 30.5 g 198.25g H₂O to 25 ml Oxidant - 50% H₂O₂ 0.69 g in deionized 4.8 g 31.2 g H₂Oto 25 ml Base - NH₄OH (28-30% NH₃) 2.29 g in distilled 16 ml indistilled 104 ml in distilled H₂O to 3.4 g H₂O to 40 ml H₂O to 260 mlDistilled water rinse 2 ml 20 ml 100 ml Precipitation Process 1.Stabilize water at 15-25° C. 2. With mild stirring, add solutions in thefollowing order: Ce(NO₃)₃, MEEA, H₂O₂ 3. Insert a Silverson mixer withappropriate ¾-in tubular mixer Standard mixer Standard mixer mixing headand jets head w/fine head w/fine head w/medium screen - 7,000 rpmscreen - 5,000 rpm screen - 8,100 rpm 4. Pump NH₄OH solution at flowrate of 17 ml/min 200 ml/min 650 ml/min 5. Rinse water purge at flowrate of 17 ml/min 200 ml/min 650 ml/min 6.. Heat the mixture to 70° C.by Placing beaker Ramping Ramping in preheated temperature temperature70° C. bath over 25 min over 25 min 7. Hold at 70° C. for 50 min 8. Coolmixture to 20-25° C. 9. Filter via diafiltration to less than 3 mS/cm,and concentrate by 20X

Particle sizes were determined for 19 of the large (9.5-liter) batchesprepared as described above. The average particle hydrodynamic diameterwas 5.8 nm, with a standard deviation of 0.40. Average particle sizesmeasured for 207-ml and 1.5-liter batches have generally fallen in therange of 5.2-6.4 nm, well within ± two standard deviations of 5.8 nm(95% confidence level). Therefore it is reasonable to conclude that theparticles from the two smaller batches are of essentially the same sizeas those of the large batches.

Example 6 Preparation of Fuel Concentrate

A portion of cerium dioxide dispersion, prepared as described in Example1, was added slowly to a mixture of D2 diesel fuel, surfactant AerosolAOT, and 1-hexanol co-surfactant, resulting in a clear reddish browncolored solution that can be employed as a fuel concentrate. Theconcentrate is 14% by volume cerium dioxide dispersion; the remainingvolume is 1.72% 1-hexanol co-surfactant, 18.92% surfactant Aerosol AOT,and 65.36% diesel D2 diluents.

Example 7 Preparation of Additivized Diesel Fuel Containing FuelAdditive

A portion of the fuel concentrate, prepared in Example 6, was diluted 1part to 600 parts of diesel fuel by volume. Thus the final additivizedD2 fuel has nominally a concentration of 42 ppm (by weight) of CeO₂ and258 ppm water and 361 ppm Aerosol AOT.

Example 8 Evaluation of Additivized Diesel Fuel

The additivized diesel fuel was evaluated in an Element Power Systemsmodel #HDY5000LXB diesel generator operating at a Frequency of 60 Hz anda Power Factor of cosφ=1.0 rated at 5 KVA (AC power output). The dieselengine is a model #DH186FGED forced air cooled 4 stroke with a ratedmaximum power output of 10 HP. A portion of the exhaust is drawn througha porous filter medium by the action of a downstream in-line vacuumpump. Diesel particulate matter is collected on the filter media for 150seconds, after which time its percent grey scale is measured (AdobePhotoshop). The percent grey scale is taken as a measure of the amountof soot collected. The grey scale level increases as the amount of sootpresent on the filter media increases.

The diesel engine was operated for over an hour using normal D2 (lowsulfur 500 ppm) fuel to equilibrate it. Towards the end of this time,diesel particulate matter was collected on a filter media for 150seconds. The percent grey scale of the filter, which correlates with theamount of particulate material present, was measured at 70%, a figuretypical for these operating conditions and collection times. The enginewas turned off; the fuel tank was drained of regular D2, and thenpartially filled and drained twice with additivized diesel fuel. Thetank was then filled to the two-thirds level with additivized dieselfuel. The engine was then operated with the additivized D2 fuel for overan hour to equilibrate it to the new fuel. An increase of 3% in theenergy output of the generator was measured (voltage multiplied bycurrent through a 1.2 KW resistive load). The engine was turned off, theadditivized fuel was drained from the fuel tank, and the tank was rinsedtwice with normal diesel fuel and then filled to the two-thirds markwith normal diesel fuel. The engine was then operated for twenty minutesto purge the lines and filters of any residual additivized fuel. A powermeasurement indicated that the engine had returned to the normaloperating conditions, that is, the 3% increase in power obtained whenthe engine was operated with additivized fuel was no longer observed,indicating that there is no residual additivized fuel in the system.Diesel particulate was collected for 150 seconds, as describedpreviously, and the percent grey scale was measured as 40%. Thisrepresents a 43% reduction in diesel particulate matter, as determinedby the change in the grey scale of the test filter, even though the fuelno longer contains additive.

This example illustrates that the internal working parts of the enginehave been conditioned by the nanoparticulate CeO₂ in a time scale ofapproximately one hour. Conditioning involves incorporating CeO₂ intothe walls and pistons of the engine. The CeO₂ is assisting in carboncombustion by providing oxygen according to the following reaction:

2CeO₂⇄Ce₂O₃+½O₂.

Improved combustion results in a reduction of particulate matter asreflected in the diminished grey scale of the test filter.

Example 9 Preparation of a Cerium Dioxide-Containing Fuel Additive ByReverse-Micelle Formation

D2 diesel fuel (2320 mL) and co-surfactant, 1-pentanol (200 mL) wereplaced in a 6 liter Erlenmeyer flask. The surfactant, AOT (800 g), whichwas broken into small particles before addition, was then added in 40 gmportions to the flask with magnetic stirring. Following addition of theAOT , the resulting clear solution was allowed to stand for 1 hour.During this time, the solution changed from a light amber color to anorange color as the microemulsion formed.

A 500-mL dispensing burette containing 525 mL of the aqueous CeO₂solution (nominal 1.0 M CeO₂ stabilized with 1.5 M MEEA) was mountedover the flask. The first 400 mL of this solution was added as a slowsteady stream with stirring. As the aqueous CeO₂ was added, a slime-likecloud surrounded the vortex. The addition was stopped every 100 mL toallow the solution to clear. Initially, the solution required about 1minute to clear between 100-mL additions, but after 200 mL had beenadded, the solution cleared more rapidly. A slower addition rate for thelast 125 mL was used; addition was stopped every 50 mL to allow thesolution to clear. Addition of aqueous CeO₂ over a 90 minute periodresults in a deep orange-brown solution, that was allowed to equilibratefor 12 hours, during which time the color had changed from orange-brownto greenish-brown.

The procedure described above was used to prepare 1.00 gallons (3.785 L)of a microemulsion containing about a 19700 ppm CeO₂ in D2 diesel fuel,with a water to surfactant (AOT) mole ratio of 16.2 and an aqueousvolume fraction of 14%. A 1:600 addition of this microemulsion to D2diesel fuel (density 0.85 g/mL) gives a fuel having 32.8 ppm ceriumdioxide (based on 70% yield of a 10× concentrate of CeO₂ prepared froman initial 0.0945 Ce(NO₃)₃ solution) having 30 ppm sulfur and 265 ppmwater.

Example 10 Test Data: Griffith Energy On-Road Tests

A “cetane improved” formulation that included reverse micellescontaining 220 ppm hydrogen peroxide and 220 ppm water suspended inultra low sulfur diesel was run at Griffith Energy from Oct. 18 to Nov.17, 2006, using both a control and test 12-liter diesel, class 8tractors. Once each week, mpg (miles per gallon) data were downloadedfrom each of the Volvo truck on board computers (“Trip Manager”) and fitto a linear regression model that explained 80% of the mpg variation.The data are presented in the table below. The greatest improvements onday 21 and day 35 are underestimates of the true potential of theformulation, as non-treated days were averaged into the weekly results,due either to beginning the treatment mid-week (day 20) or encounteringfilter plugging (day 28). Chemical analysis of the plugged fuel filtersrevealed primarily soot particles, from which it can be concluded thatthe formulation cleans all of the engine parts, including the fuelcirculation system. No data were collected on day 49, but it is believedthat the treated truck was becoming “dirty” (normal operation), and thatday 49 or subsequent data would have shown that this truck returned tothe baseline of 4.74 mpg. Based upon the mpg baseline offset of 1.72%,the cetane improved formulation demonstrated a maximal effect of 9.44%improvement in mpg (day 35)

Control (mpg) Experiment (mpg) Percent Change Day 4.42 4.74 1.72 start4.75 4.74 1.72 7 4.66 4.84 3.86 14 4.57 5.05 8.37 21 4.76 4.76 2.15 284.83 5.18 11.16 35 4.66 5.00 7.30 42

Example 11 Static Engine Test Data

Test Data: Environmental Energy Technologies (EET) Static EngineTest-EET diesel generator specifications are as follows:

Generator: Element Power Systems model # HDY5000LXB Frequency 60 HzPower Factor cos φ = 1.0 Rated AC Output 5 KVA Engine: model # DH186FGEDType forced air cooled 4 stroke Max Output 10 HP Fuel diesel light fuel(BS-AI) Fuel Consumption Rate 210-286 g/kW Oil Temperature <95° C.Exhaust Temperature <480° C.

The diesel generator tank was drained and flushed of old fuel two timesbefore refueling with new D2 diesel fuel. The engine was brought to asteady state at the beginning of each day's test by running at 30% loadfor a warm-up period of approximately 10 to 20 minutes, which alloweddrainage of old fuel from the engine fuel. Following warm-up, testingwas performed for the given load by drawing exhaust at a fixed flow ratethrough filter papers for a duration of 150 seconds per sample. Anestimate of diesel particulate matter (soot) and the effect of the fuelformulation was made by measuring the optical reflectance of the filterpaper that had entrained the soot. Between fuel changes, the engine wasgiven approximately 5 minutes to reach steady state operatingconditions. For tests requiring the fuel additive, the engine was turnedoff, drained and flushed twice with premixed fuel containing the fueladditive emulsion.

The data in the table that follows indicate that, at 1500 ppm water, thediesel generator power drops from 1080 w to 320 w, a decrease of 70% fora drop of 5° C. This is accompanied by a 16% reduction in dieselparticulate matter, clearly a very poor power for pollution trade-off.

Subsequent testing revealed that as much as 300 ppm of water had verylittle if any effect on power while reducing diesel particulate matterby 13%. Finally, as much as 28% of the diesel particulate matter can bereduced by a very substantial concentration of water, 960 ppm with onlya small 5% power loss when the formulation contained 540 ppm of hydrogenperoxide. Thus by balancing the water effect of lowering combustiontemperature/efficiency and soot production by the presence of a freeradical initiator such as hydrogen peroxide it is possible tosimultaneously maintain high engine performance and achieve a loweringof the DPM thereby avoiding a power for pollution trade-off.

Total PM Reduction water/ Load Exhaust Comparison H2O2 decane AOT Date(watts) T ° C. to Control ppm ppm ppm Test 1 D2 Control Jul. 6, 20061080 w 103 C.  0% 0/0 0 0 Test 2 Emulsified D2  320 w  98 C. 16%1500/0   5100 3400 Test 1 D2 Control Jul. 18, 2006 1217 w 106 C.  0% 0/00 0 Test 2 1224 w 113 C. 13% 300/0  1020 680 Test 4 1150 w 114 C. 28%960/540 5100 340

Example 12 Formulation of a Stable Non-Sulfur-Containing Free RadicalReverse-Micelle Composition

130 ml of a 1-hexanol solution is added with low shear mixing to 440 mlof Ultra Low Sulfur Diesel. Then 310 ml of Triton N-57 is added to thediesel alcohol mixture. A gestation time of 1 hour is allowed. Finally,120 ml of a 50% hydrogen peroxide/water solution is added to the abovemixture at a constant flow rate over a 15 minute period allowing forgood uniform volumetric mixing during this time period. After a 12 hourequilibration period the micro emulsion has reached a statecharacterized by particles that are measured to be in the range of 5 nmto 9 nm (by light scattering). This concentrate diluted one part per 500would give a final concentration of 120 ppm H₂O₂ and 120 ppm water.

Example 13 Preparation of Reverse-Micelle Free Radical Initiator UsingStabilized Hydrogen Peroxide

The following formulation makes 1.0 L of a 12.7 v % of a 50 w % aqueoushydrogen peroxide solution stabilized in a Triton N57/1-hexanol/dieselmicroemulsion. This formulation, when diluted 1/500 in ultra low-sulfurdiesel, will contain 150 ppm (mg/L) H₂O₂ active ingredient and 150 ppm(mg/L water).

To 435 mL ultra low-sulfur diesel fuel in a 1.5 liter vessel is added113 mL 1-hexanol, with good volumetric stirring, until a homogeneousmixture is formed. Then, 325 mL of the non-ionic surfactant Triton N57is added, with good mixing. After one hour, which enables thethree-component mixture to stabilize, 127 mL of 50 wt % aqueous hydrogenperoxide is slowly added over a 15 minute period. The aqueous hydrogenperoxide had been previously stabilized against catalytic decompositionby free metals with stannate and metal chelating agents, e.g.,phosphonates and/or etidronic acid.

A period of twelve hours is allowed for the final emulsion to reachthermodynamic equilibrium. Samples of this microemulsion, diluted both1:250 and 1:500 parts with ULSD, are stable down to 5° C., with noapparent chemical degradation, and stable for 2.5 hours at 100° C.without apparent oxidation (as determined by UV/visible spectroscopy).

Example 14 Reverse-Micelle Free Radical Initiator Composition Containingt-Butyl Hydroperoxide and 70% Neutralized Oleic Acid

The following formulation makes 1.0 L of a 30 v % t-HYDRO solution(tertiary butyl hydroperoxide) stabilized in a oleicacid/ethanolamine/1-hexanol/diesel microemulsion. This formulation, whendiluted 1/500 in ultra low-sulfur diesel will contain 390.6 ppm (mg/L)active ingredient (t-butylhydroperoxide).

To 418.0 mL of ultra low-sulfur diesel fuel in a 1.5 liter vessel isadded 35.0 mL 1-hexanol, with good volumetric stirring, until ahomogeneous mixture is formed. Then, 220.0 mL of technical grade oleicacid is added, with good mixing, followed by 27 mL of ethanolamine.After one hour, which enables the four component mixture to stabilize,300.0 mL of t-HYDRO (70 v % t-butyl hydroperoxide in water) is slowlyadded over a 25 minute period, preferably at a temperature above 25° C.

A period of twelve hours is allowed for the final emulsion to reachthermodynamic equilibrium. Samples of this microemulsion diluted 1:250parts with ULSD are stable down to 5° C., with no apparent chemicaldegradation, and stable for 2.5 hours at 125° C. without apparentoxidation (as determined by UV/visible spectroscopy).

Example 15 Improved Lubricity Using Fuel Including Cerium DioxideParticles

Lubricity was determined by measuring wear on a ball bearing rubbed on aplate coated with fuel containing the respective fuel additives. Wearwas determined by the depth, in mm, of the average scar imparted byrubbing. Neat fuel, without an additive, gave a 0.35 mm scar. Testresults for fuel with a commercial additive, Platinum Plus™; acomparative fuel additive including 10 nm particles; and the inventivefuel additive including 5 nm particles were 0.32, 0.31, and 0.245 mmrespectively. Low wear numbers correlate with greater lubricity. Thus,the inventive small particles afford a 30% improvement in lubricity.

While the invention has been described by reference to various specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

1. A method of improving the efficiency of a diesel engine provided witha source of diesel fuel, said method comprising the steps of: a) addingto said diesel fuel a reverse-micellar composition comprising: i) anaqueous first disperse phase comprising a free radical initiator; andii) a first continuous phase comprising a first hydrocarbon liquid, afirst surfactant, and optionally a co-surfactant, thereby producing amodified diesel fuel; and b) operating said engine, thereby combustingsaid modified diesel fuel.
 2. The method according to claim 1, whereinsaid diesel fuel is selected from the group consisting of D2 diesel, lowsulfur diesel, ultra low sulfur diesel, and biodiesel.
 3. The methodaccording to claim 1, wherein said free radical initiator is selectedfrom the group consisting of stabilized hydrogen peroxide, t-butylhydroperoxide, and mixtures thereof.
 4. The method according to claim 1,wherein said first hydrocarbon liquid comprises a hydrocarbon containingabout six to about twenty carbon atoms.
 5. The method according to claim4, wherein said first hydrocarbon liquid is selected from the groupconsisting of toluene, octane, decane, D2 diesel fuel,low sulfur diesel,ultra low sulfur diesel, biodiesel, and mixtures thereof.
 6. The methodaccording to claim 1, wherein said first surfactant and saidco-surfactant contain only the elements C, H, and O.
 7. The methodaccording to claim 1, wherein said modified diesel fuel contains lessthan about 500 ppm water.
 8. A method of improving the efficiency of adiesel engine wherein said engine is provided with a source of dieselfuel and a source of lubricating oil, said method comprising the stepsof: a) adding to said diesel fuel a reverse-micellar compositioncomprising: i) an aqueous first disperse phase comprising a free radicalinitiator; and ii) a first continuous phase comprising a firsthydrocarbon liquid, a first surfactant, and optionally a co-surfactant,thereby producing a modified diesel fuel; b) adding to the lubricatingoil a stabilized nanoparticulate composition of cerium dioxide, therebyproducing a modified lubricating oil; and c) operating said engine,thereby combusting said modified diesel fuel in said engine andlubricating said engine using said modified lubricating oil.
 9. Themethod according to claim 8, wherein said free radical initiator isselected from the group consisting of stabilized hydrogen peroxide,t-butyl hydroperoxide, and mixtures thereof.
 10. The method according toclaim 8, wherein said stabilized nanoparticulate composition comprisescerium dioxide nanoparticles having a mean hydrodynamic diameter ofabout 1 nm to about 15 nm.
 11. The method according to claim 10, whereinsaid stabilized nanoparticulate composition comprises cerium dioxidenanoparticles having a mean hydrodynamic diameter of about 6 nm.
 12. Themethod according to claim 8, wherein said first surfactant and saidco-surfactant comprise only the elements C, H, or O.
 13. The methodaccording to claim 8, wherein said reverse-micellar composition includesan alcohol as a co-surfactant.
 14. The method according to claim 8,wherein said modified diesel fuel contains less than 500 about ppmwater.
 15. The method according to claim 8, wherein saidreverse-micellar composition further includes an aqueous second dispersephase.
 16. A method of improving the efficiency of a diesel engineprovided with a source of diesel fuel, said method comprising the stepsof: a) adding to said diesel fuel a first reverse-micellar compositioncomprising: i) an aqueous first disperse phase comprising boric acid ora borate salt; and ii) a first continuous phase comprising a firsthydrocarbon liquid, a first surfactant, and optionally a co-surfactant;and b) operating said engine.
 17. The method according to claim 16,wherein said diesel fuel is selected from the group consisting of D2diesel, low sulfur diesel, ultra low sulfur diesel, and biodiesel. 18.The method according to claim 16, wherein said first hydrocarbon liquidcomprises a hydrocarbon containing about six to about twenty carbonatoms.
 19. The method according to claim 18, wherein said firsthydrocarbon liquid is selected from the group consisting of toluene,octane, decane, D2 diesel fuel, low sulfur diesel, ultra low sulfurdiesel, biodiesel, and mixtures thereof.